you should first made a screenshot you wish to use (U probably have one allready).
And you should download Adobe Phoshop (CS or something you like)...
You should resize if you want but when you save as a jpeg you should reduce the quality a litle bit... The size will decrease a lot but the quality won't reduce that much (in my oppinion)...
Hope this wasn't useles information at all... But in case that..
(download link: "http://www.download.com/Adobe-Photoshop/3000-2192_4-10255309.html?tag=lst-0-1")