I came across a great tool today, called Cropper. It makes screenshot creation so much easier. When launched, a translucent box appears that can be resized over the area of the screen that needs to be, er, "shot", and Cropper saves it as an image file on disk, all nicely cropped to the right size. The desired file format can be selected too, saving one more faff involved in the usual process.
Anyway, I highly recommend it - especially as it's free. Go try it out.