Not sure what would cause this... it could well be an attack. You could try and make a construct an html page with the png to see what it looks like (visually) or try to see if you can unbase64 encode it using a utility to see what the contents are (without running it on a browser).
I've seen similar 404 errors after a visitor arrives as the result of a normal Google search.
The wanted page and its associated images are downloaded normally.
"GET /url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAADgAAAAOCAYAAAB6pd..... 404
The very long base64 code starting iVBORw0... can be decoded to this rectangular image:
The icon to the right hand end is a Google icon, so the phenomena may have something to do with Google.
Best regards, Eric.
Author and all-around good cowpoke
Joined: Mar 22, 2000
AHA! That has got to be it - maybe Google made some sort of change around October 23 which generates this spurious GET or some other site is leaching off of Google and screws up the link creation.