Possible to use images for form buttons via css and edit just the theme files?

I have a number of form buttons (Submit, Search, etc.) that I want to change from the normal buttons into images. However, I don't want to touch the code of the module that gives the buttons (ex. search). Is it possible to just edit the theme files to convert these form buttons into images? If yes, could someone point me to the direction on how to do it? :)