photo editing system.

ok,I have a really old comp,like 3 yr old.. thats old..ok, back to the issue, This cimputer is old (;)) and it doesn;t really have picture options, all it has it .ART (never hear of it and photobucket doesnt recognize it.)and .Bmp, but it doesnt work,and wondering if anyone knows the name of a small picture editing system, that wont take too much room on a HD. I dont really want some big photo editing, icon making thing, just something small that will allow me to change up the photo type and upload it to my photobucket. Thanks!