well first you have to take a screen shot of whatever on a PC i think you have to press the print screen and paste into paint... for mac there is a widget called the screenshot plus its very handy for taking screen shot. After that you need to upload it to the web like use photobucket or something like that those sites usually already give you the code to add the pic but if they dont right click and select copy picture address and on the reply screen or sig screen press the Img button on the bar under the subject and paste the address of the picture between the two Img things
hope that helped