I found the man file for import exceptionally hard to understand. After fiddling for a while, I found that this worked well:
me:$ import -quality 50 -pause 5 screenshot.jpg
-quality determines the compression level from 1 (lowest quality) to 100 (highest quality). I used 50 for tolerable quality and small file size. Adjust according to your needs.
-pause is a delay in seconds before the screenshot will be taken. I used 5 because that gave me enough time to bring to the front the window I wanted. When import is about to take the shot, you'll see a cross cursor on screen. If you select a screen area by enclosing it in a rectangle, that's what you'll get for your shot. If you simply click on the screen, you'll get a shot of the whole window you click on. If you want parts of several windows and maybe the desktop background, you have to wait for the cross cursor to appear and then make your selection rectangle. Once you release the mouse, the picture will go into your home directory unless you include a path in the file name.
There are tons more options for import, but I didn't understand them despite reading the online manual at the ImageMagick Web site. But the command line above, modified to your needs, will give you a basic screen shot. You can also choose the png format or other image formats by using the extension with the file name. Jpg and png are probably best for Internet posting.
This is a pretty neat program because it's very easy to use and can easily select the whole screen, just the window, or parts of the window or screen.