You can get a screen shot of your whole screen at any time by pressing Print Screen. You can also use Snagit, which I personally love. However, if you'd like to do screen shots with GIMP, here's how.
1. Get ready to take the shot, then choose File > Create > Screen Shot.
2. Choose to grab a single window. Also, if you need some time after grabbing the window to do stuff, specify a delay. For instance, you might want to click on a window, then start a video, or select some text, or something else that can't be done while clicking on the window you want. (Some quick keys or cursor capture stuff is hard to do without a delay.)
3. Click Grab.
4. Drag the actual little crosshairs to the window you want. You don't see it here but when you do drag the crosshairs, a darker black border will appear around the selected window.
5. A new untitled document will appear, containing a screen shot of the selected window.
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