Problem: you have a huge image and you want to resize the is so that it will display nicely on a web page. The example image is this beauty m51_hallas_big.jpg from APOD [APOD03].

The first thing that you might notice, is that GIMP opens the image at a logical size for viewing. If your image is really big like the sample image, GIMP will set the zoom so that it will display nicely on the screen. The zoom level is shown in the status area at the bottom of the Image window.

The other thing to look at in the title-bar is the mode. If the mode shows as RGB in the title bar, you are fine. If the mode says Indexed or grayscale, read the Section 4.6, “Change the Mode”.

Use Image → Scale Image to open the Scale Image dialog. You can right click on the image to open the menu, or use the menu along the top of the Image window. Notice that the Scale Image menu item contains three dots, which is a hint that a dialog will be opened.

If you have a desired width, put it in the dialog at the top where it says Width. If you don't have such a number in mind, you can steal the width of GIMP's default image size, which is 256 pixels. This is shown in the figure above.

Perhaps you want your image to look more like a 4x6 inch photo on most image rendering web browsers. Switch the units to “inches” and enter 4 inches in the height box (opting for smaller than 4x6 rather than bigger). You can see this dialog above.

Let GIMP choose the other dimension length for you. Meaning, it requires more image knowledge to change both width and height and have it look correct. So change only one item and let GIMP change the rest. To change the other dimension, see Section 4.4, “Crop An Image”.