I hope you're saving the mime-type value of every image that you save to database (mime type is reported by the request during upload). If you don't, you'll have to look up what a mime type would be based on image-file extension or its content format. You'll need this value when images are requested (from within CSS files).
Implement an HTTP handler (.ashx) specifically for these images (new file > generic handler > fill up ProcessRequest).
Then, replace url(logo.gif) in your css files with something like this:
url(/GetImage.ashx?user=1234&image=logo);
Obviously, you can have the filename and query to be whatever you like/need. Maybe you have user info in session, so you don't have to include user id there. Maybe you'll want to implement this handler only for logo images, and if you have user info in session, you can simply say (css):
url(GetLogo.ashx);
Then, within that ashx handler, write code to get the image from the database (or where ever it is), and stream it to the browser. Something like this:
byte[] imageData = ... // get this from db
MemoryStream ms = new MemoryStream(imageData); // System.IO namespace
Response.ContentType = "image/png"; // remember the mime-type?
ms.WriteTo(context.Response.OutputStream); // context is current HttpContext
Alternatively, you can inject the whole image into the CSS, by encoding it to base-64 format. Note that the size of css file will grow for the size of the image, and you're practically combining multiple downloads into 1, which may be a good or a bad thing.
Example below (google's logo); paste this in CSS (notice the size of that thing).
Besides CSS, the data in this example (data:image/png;base64,iVBORw0KGg ...) can be used as URL with IMG tags, or manipulated by JS.
.somediv
{
width:275px;
height:95px;
background-image: url(data:image/png;base64,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);
}
+1 if you like this :)