1) Can't really do that on most shared hosts.
2) Sprites are multiple images combined into one large image. This large image should be equal to or, preferably, less than in size than the combined size of all the images it consists of. The reason why this is faster is because it reduces HTTP requests. Browsers have a limit on the amount of parallel downloads they can make, so by combining images into a sprite you can download more things faster. vBulletin does not support CSS sprites atm and it would be a pain in the ass to make a modification that does.
3)
http://hellboundbloggers.com/2011/06...ve-site-speed/
Google if your friend, especially for number three.