As imagens são as entidades básicas utilizadas pelo GIMP. A grosso modo, uma “imagem” corresponde a um único arquivo, como um arquivo TIFF ou JPEG. Você também pode pensar em uma imagem como correspondendo a uma única janela de exibição (embora, na verdade, é possível ter várias janelas, todas exibindo a mesma imagem). Não é possível ter uma única janela exibindo mais do que uma imagem ou, no entanto. para uma imagem não existir janela exibindo-a.

A imagem no GIMP pode ser uma coisa muito complicada. Em vez de pensar nela como uma folha de papel com uma imagem sobre ela, é melhor pensar nela mais como uma pilha de folhas, com cada folha chamada de “camada”. Além de uma pilha de camadas, uma imagem do GIMP pode conter uma máscara de seleção, um conjunto de canais e um conjunto de caminhos. Na verdade, o GIMP fornece até um mecanismo para agregar dados arbitrários, chamados “parasitas”, em uma imagem. Mas só extensões especializadas fazem uso desse mecanismo.

No GIMP, é possível ter várias imagens abertas ao mesmo tempo. Embora as imagens grandes podem usar muitos megabytes de memória, oGIMP usa um sofisticado sistema de gestão de memória baseado em pedacinhos (tiles) que permite ao GIMP lidar com imagens muito grandes de forma elegante. No entanto, há limites para isso, e ter mais memória disponível pode melhorar o desempenho do sistema.

If a simple image can be compared to a single sheet of paper, an image with layers is likened to a sheaf of transparent papers stacked one on top of the other. You can draw on each paper, but still see the content of the other sheets through the transparent areas. You can also move one sheet in relation to the others. Sophisticated GIMP users often deal with images containing many layers, even dozens of them. Layers need not be opaque, and they need not cover the entire extent of an image, so when you look at an image's display, you may see more than just the top layer: you may see elements of many layers. For more info see Introduction to Layers.

Digital images consist of a grid of square pixels. Each image has a size measured in two dimensions, such as 900 pixels wide by 600 pixels high. But pixels don't have a set size in physical space. To set up an image for printing, we use a value called resolution, defined as the ratio between an image's size in pixels and its physical size (usually in inches) when it is printed on paper. Most file formats (but not all) can save this value, which is expressed as ppi—pixels per inch.

When printing a file, the resolution determines the size the image will have on paper, and as a result, the physical size of the pixels. The same 900×600 pixel image may be printed as a small 3×2" card with barely noticeable pixels—or as a large poster with large, chunky pixels.

Images imported from cameras and mobile devices tend to have a resolution attached to the file. The resolution is usually 72 or 96ppi. It is important to realize that this resolution is arbitrary and was chosen for historic reasons. You can always change the resolution with GIMP—this has no effect on the actual image pixels. Furthermore, for uses such as displaying images online, on mobile devices, television or video games—in short, any use that is not print—the resolution value is meaningless and is ignored. Instead, the image is usually displayed so that each image pixel conforms to one screen pixel.

Um Canal é um único componente de cor de um pixel. Para um pixel colorido no GIMP, esses componentes são geralmente vermelho, verde, e azul (R,G,B), e, algumas vezes, a transparência (Alfa). Para uma imagem em Tons de cinza, os canais são Cinza (ou Valor) e Alfa e para uma imagem colorida indexada, eles são "índice" e alfa.

The entire rectangular array of any one of the color components for all of the pixels in an image is also referred to as a Channel. You can see these color channels with the Channels Dialog.

Quando a imagem é exibida, o GIMP combina estes componentes para formar as cores de pixel para a tela, impressora ou outro dispositivo de saída. Alguns dispositivos de saída podem utilizar canais diferentes de vermelho, verde e azul (RGB). Se o fizerem, os canais do GIMP são convertidos no mais adequado para o dispositivo quando a imagem for exibida.

Canais podem ser úteis quando você está trabalhando em uma imagem que precisa de ajuste em uma determinada cor. Por exemplo, se você deseja remover “olhos vermelhos ” a partir de uma fotografia, você pode trabalhar somente no canal vermelho.

You can look at channels as masks which allow or restrict the output of the color that the channel represents. By using Filters on the channel information, you can create many varied and subtle effects on an image. A simple example of using a Filter on the color channels is the Channel Mixer filter.

Além destes canais, o GIMP também permite criar outros canais (ou mais corretamente, máscaras de canais), que são exibidos na parte inferior da caixa de diálogo de Canais. Você pode criar um Novo canal ou guardar uma Seleção para um canal (máscara). Veja a entrada do glossário em Máscaras para obter mais informações sobre máscaras de canais.

Often when modifying an image, you only want a part of the image to be affected. The “selection” mechanism makes this possible. Each image has its own selection, which you normally see as a moving dashed line separating the selected parts from the unselected parts (the so-called “marching ants”). Actually this is a bit misleading: selection in GIMP is graded, not all-or-nothing, and really the selection is represented by a full-fledged grayscale channel. The dashed line that you normally see is simply a contour line at the 50%-selected level. At any time, though, you can visualize the selection channel in all its glorious detail by toggling the Quick Mask button.

A large component of learning how to use GIMP effectively is acquiring the art of making good selections—selections that contain exactly what you need and nothing more. Because selection-handling is so centrally important, GIMP provides many tools for doing it: an assortment of selection-making tools, a menu of selection operations, and the ability to switch to Quick Mask mode, in which you can treat the selection channel as though it were a color channel, thereby “painting the selection”. For more information, see also The Selection.

When you make mistakes, you can undo them. Nearly everything you can do to an image is undoable. In fact, you can usually undo a substantial number of the most recent things you did, if you decide that they were misguided. GIMP makes this possible by keeping a history of your actions. This history consumes memory, though, so undoability is not infinite. Some actions use very little undo memory, so that you can do dozens of them before the earliest ones are deleted from this history; other types of actions require massive amounts of undo memory.

You can configure the amount of memory GIMP allows for the undo history of each image, but in any situation, you should always be able to undo at least your 2-3 most recent actions. The most important action that is not undoable is closing an image. For this reason, GIMP asks you to confirm that you really want to close the image if you have made any changes to it. For more information, see also Undoing.

A lot of the things that you do to an image in GIMP are done by GIMP itself. However, GIMP also makes extensive use of “plug-ins”, which are external programs that interact very closely with GIMP, and are capable of manipulating images and other GIMP objects in very sophisticated ways. Many important plug-ins are bundled with GIMP, but there are also many available by other means. In fact, writing plug-ins (and scripts) is the easiest way for people not on the GIMP development team to add new capabilities to GIMP.

Todos os comandos do menu Filtros, e um número substancial de comandos em outros menus, são implementados como plug-ins.

In addition to plug-ins, GIMP can also make use of scripts. Scripts are written in a language called Script-Fu, which is unique to GIMP (for those who care, it is a dialect of the Lisp-like language called Scheme). In the past there was a clear distinction between scripts and plug-ins, but that is disappearing. Depending on which Script-Fu interpreter you use, Scheme scripts can also be installed as plug-ins.