help request for dynamic graphics

Hi guys, I hope someone can point me in the right direction. First I want to add some custom fields to the settings-general tab. What I want is for user to be able to paste a url to a graphic. Then have it save as a variable. I changed my style.css to php and the header is refrencing it ass css still with a couple of commands I learned. I want to be able and go through and change the certain graphics in the style sheet to viarables such as $mainpagebackground, $columnbackground, $headergraphic, and so on. Is there any tutorials out that could help me accomplish this or someone could be nice enough to help me work through this?