I think I might be the only one to have complained about this so far, but the fact that Telerik decided to use App_Data to store things like css, images, templates is a massive pain in the ass and this issue is just one of those pains. If you want to display an image using standard methods you need to move your images outside the App_Data folder. I think the only other way to do it is to use Sitefinity Handlers in order to get access to items in that folder. As far a I'm concerned using App_Data for assets is a huge list of drawbacks and not a single benefit.
I've now actually moved all images to /images/ folder at the root and in my css files I just reference them as url('/images/....') I can't prove it as I haven't done enough testing but I think there's also a performance hit by having SItefinity process and server up images from the App_Data folder vs. just having them in the root of the site and not touched by a handler.