In all edition programs, you can find selection tools that allow you to pick a part of the image to apply changes (colour, filters, size...) locally, independently from the rest of the picture.
At this point, you may already know that you can make a rectangular or elliptic selection with GIMP, by choosing the direct commands from the icons in the tools window, or rather, by pressing the keyboard shortcut keys (in this case, "R" and "E", respectively.
Furthermore, you have a third selection tool for freehand marking (Key "F") and the uniform colour selection ("U"), also called "magic wand" by some people.
For the rectangular and elliptic selection tools, you have two useful key combinations, that can be applied simultaneously:
- With the "Alt" key pressed, selection will be done from the center of the shape.
- With the "Caps" key pressed, the selected shape will be a square or a circle, same width than height.
- If you press both keys in the same action, the selection will be a square (or circle) drawn from its center.
One step further, you can combine several selection tools to mark complex areas. The only condition is NOT to unselect between the use of one tool and the next one. The way to do this will also be the use of keyboard shortcuts.
If you have an area selected (for example, an square), you can add a new one to it (a circle?) by keeping the square selection active and selecting the circle with the "Caps" key pressed.
You may repeat this process as many times as you need (also changing the selection tool, if needed) to get the final selection.
If what you want is to remove some specific content from an existing selection, the process is similar, but in this case you have to press the "Ctrl" (or "Command" in Macs) key.
By adding and removing selections, you can easily reach the final planned area (depending on the image complexity). This way, you will be able to remove backgrounds, use the subject in a composition, or just edit it with some colour or filter correction.
And a secret...
Even when the option is also available in other edition programs, GIMP adds one specific icon (and command) as a shortcut to a very useful tool: the selection by specific colour (Key "O").
It does just that - It picks, out of an image, all areas with a same colour. It would be the same as using the magic wand several times, to select separated areas (for example, in the background) to reach the desired selection...
Check the following post: Advanced selection with GIMP