Ptint Scr will save an image of your desktop. Alt + Print Scr (or Fn + Alt + Print Scr on most laptops) will save an image of the active program. Paste it into any image editing program. MS Paint..or any other. From there edit, save, and post.
A better solution is to highlight the spreadsheet area ONLY that you want to copy, and Ctrl "C" (copy) to Paint. No need to copy all the extraneous toolbars, etc. You will want to save the Paint file to a format that is allowed here, don't use the bitmap format, it sucks. See my chart in the draft tracking thread for an example.