So it took me a while to figure this one out, therefore thought it would be worth posting here…
Facebook currently allows only one url type declaration per style block. So if you have multiple url declarations in your style, you get this error. The solution is to simply seperate the url declarations into separate style blocks:
<style type="text/css">
#div1 { background: url(http://www.imageserver.com/img1.jpg); }
#div2 { background: url(http://www.imageserver.com/img2.jpg); }
</style>
To become:
<style type="text/css">
#div1 { background: url(http://www.imageserver.com/img1.jpg); }
</style>
<style type="text/css">
#div2 { background: url(http://www.imageserver.com/img2.jpg); }
</style>
