Firstly, thank you for creating such a great plugin!
I'm having a couple of troubles with my site http://www.documentarytube.com if you browse to documentary with comments you'll notice that some of the avatars are being displayed, and others not (ie. http://www.documentarytube.com/the-century-of-the-self).
I'm using wordpress 3.2.1, I've already changed the permissions of the /cache directory to 777 and have checked that the image is present and loading correctly on my site which it is.
If I type in the generated image URL
it states the following:
The type of image you are trying to process is not allowed
Thanks again for any help you can offer,