if you have a mac, hold down shift-command-4 and then select the area you want to take a pic of. if you want to select the whole window of a program, do the same, but instead of selecting an area, press the space bar, and then click on the program you want to get the pic of. the pic will save automatically as "Picture 1,2..) unfortunately, FM saves as PNG, which is usually to big, but if you open the picture in Preview, you can then "Save As", select JPG, and then move the "Quality" slider until the file size is less that 300 KB.
there might be a built-in way to do the same thing, but i don't know it unfortunately