<ALT>+<PrintScreen>
Open Paint. Edit menu. Paste. Save it. This will save as <filename>.bmp.
You can attach that but it won't display like you usually see here. To get it to show when the thread page loads, it'll need to be converted to .jpg. Adobe PhotoShop is what I use for this, but if you don't have it or something similar, check the popular download sites like zdnet, cnet, or tucows for an image format conversion program. Have fun!
Forgot to add, if you happen to want a full screen capture just ignore the <ALT> button.
<PrintScreen> = full screen.
<ALT>+<PrintScreen> = Active window.