How can I block all inline use of images from my sites, the way I block conventional hotlinking to images through .htaccess?

OK, I admit I read error logs, and lately I've been getting a lot of errors that begin like this...

[Mon Jan 24 14:09:38 2011] [info] [client XX.XXX.X.XX] found %2f (encoded '/') in URI (decoded='/skinny/url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAADgAAAAOCAYAAAB6p...

The error always refers to pages on my site, but here's the weird deal. There are no PNG images on the referring page, there are no spaces (%2f) in any file name on the page, image or otherwise.

So now my questions:

1) What's going on here?

2) Is this a hack attempt?

3) Is this somebody trying to take content and run it inline somewhere else on their site?

4) How can I block all inline use of images from my sites by other sites, the way I prevent all convention开发者_如何学Pythonal hotlinking to images through .htaccess?

This looks like a broken image reference in a CSS style sheet.

The url(data:image/png;....) part is using the Data: URI scheme which is used for embedding image data directly in style sheets or HTML pages (instead of having a separate image resource). However, it seems to be used in connection with a physical location somehow, which is wrong.

The access log should have a referer column that shows you which page or style sheet is the culprit.