First of all, great plugin!!
Here's my problem. Some last.fm albums have question marks where the cover images are supposed to be on the last.fm website (one example is this: http://www.last.fm/music/Mew/Frengers). Last.fm Records can't find those images and so I get the question mark on my website too. When I try to open the (missing) image in a new window, I get a 404 error (as expected) and the url in this format: http://cdn.last.fm/coverart/130x130/3414792-1098281393.jpg. All other images that are pulled successfully are cached.
I've only been given the option to upload an image for a missing album cover once, and that was for an album that is actually not uploaded on the Last.fm website. So, is it possible to update the plugin to check for those errors and offer the options to upload my own image for the missing cover?