Don't reply so fast... lol... I was making a screenshot to show you
When you create a post, at the bottom you can browse for a file to attach. So, just press the browse button and browse for your screenshot.
Also, make sure that you don't post a .BMP screenshot, but convert it to a JPG or GIF first in your grpahics editor. Also, crop the screenshot before you attach it, not the entire screen is needed...
Furthermore, can't you just type over the message that it says?