The most obvious way to open an existing image is the Open Image dialog. Use File → Open…, or Ctrl+O to open the Open Image dialog. This dialog allows you to navigate to the image you want to use, select its name, and then use the Open button to load the image. For details on how to use this dialog, see the above link.

The Open Image dialog provides several features to help you navigate quickly to a file. Perhaps the most important is the ability to create “bookmarks”, for folders that you use often.

You can directly open an image from an online or network location if you know the URI (i.e., a web or network address) for the image. Select File → Open Location… from the main menu. For more details, see the Open Location… documentation.

To open an image that was recently used in GIMP, select File → Open Recent from the main menu. This displays a submenu of the most recently opened images that you can click to open the desired image. At the bottom of this submenu is the Document History command if you need to reopen an image from longer ago. See also Open Recent.

If you have associated an image file type with GIMP, either when you installed GIMP or later, then you can navigate to the file using a file manager (such as Nautilus or Konqueror in Linux, or Windows Explorer in Windows), and once you have found it, double-click on the file. If properly configured, the image will open in GIMP. Most file managers also support drag and drop (see below).

You can “Drag and drop” one or more images onto the Toolbox, or, if you haven't opened any images yet, the empty image window, to open the dragged image or images.

You can also drag one or more images into an open GIMP image. The dropped image or images will be added to the open image as a new layer, or set of layers.

Many applications support dragging and dropping an image into GIMP; for example, drag an image from Firefox and drop it onto GIMP's toolbox.

Use File → Create → From Clipboard (see From Clipboard) to create a new image from the clipboard; alternatively, you can use Edit → Paste as → Paste as New Image (see Paste as New Image). Many applications support copying an image to the clipboard that can then be pasted into GIMP. Many operating systems support copying screen content to the clipboard.

Print Screen typically copies the screen content to the clipboard. Print screen is not universally supported, and just because your operating system can copy an image to the clipboard, does not mean that GIMP can use the image from the clipboard. Your best bet is to try it and see if it works.

There are many image management applications that can be used to browse and manage your images. They usually work similar to a normal file browser in that you can double click images to load them in their default application; or you can use drag-and-drop to drop one or more images on GIMP's toolbox.

Example applications on Linux are: gThumb; see the gThumb website [GTHUMB]. Another similar application is Geeqie [GEEQIE].