I'm not really sure why it's downsized. In any case, just make sure that if you do get it bigger, it's less than or equal to 150px high and 650px wide.
Upload it to a host sight (Imageshack, Photobucket..) then use the image link from there. Uploading straight from your PC means that the pic will be resized here on the forums.
hmm well its a pic that i customized on paint then saved it, so.. maybe i have to get photobucket or something and put it on that?
Rise and rise again, until lambs become lions