You do this in CSS. PHP is a server side language, it doesn't parse images/styles. And anybody can get the image URL through Firebug or something similar. The only site that I haven't been able to get an image URL for is eBay.
No, it doesn't. Your images can be stolen all the time. Also, CSS doesn't really stop image steals, never even heard of that ever being able to. There are ways to not hot-link files. I would search more on that, this does probably require PHP in some cases, I don't really bother with that. But really search for hot-linking and stuff.
You could use CSS and then find a way to protect your css file, or directory. Other than that, its impossible. There are hundreds of ways to obtain your site's images.
Also, why don't you want them clickable? I mean, you would either have unique images which could be used somewhere else (like a logo), or a pretty general image which could be found anywhere.