Assume you have windows.
If so;
1- make the window you want to capture the active window.
2-hit Alt-PrtScn
3-Open Paint.
4-Paste the clipboard (Ctrl-v, or right click paste)
5-Save the file as a JPG
In your post, go to manage attachments in the box below the new post window, click 'Manage Attachement'.
Click browse, find the file (if you forgot where you put it, try the 'Recent' files). Highlite it, and click upload. It should take a few seconds. Once you see your filename under the manage attachments (possibly above) button, just drag it up into the new post window.
WaLa!