Tricks for creating fallback background-image?

The character sheet I maintain recently had a background image disappear because the server went down. I'm curious to know if there is a mechanism for creating some kind of fallback image or possibly color? I've done a bit of googling around, but there is no direct way that I could find within css3. It looked like you could comma separate values, but the simple test I didn't turn out exactly the way I expected. Is there a generally accepted way of getting around this css3 limitation?

You could put the whole sheet in a div, and assign a second image to that one (perhaps with a z-index), so when the first disappears, the second will show through. Now, the sheet will still load both images, just one on top of the other. This will increase the bandwidth requirement, which might irritate the Power That Be (tm). A better option is to have your own website or account on a website that's not coming down anytime soon to host your images, and link the sheets to that. However, I would caution care against putting images up that don't belong to you and are not public domain. Folks put effort in to making art; let them get their due, or at least get permission for things with owners.