I recently started getting this error from Gravatar “The type of image you are trying to process is not allowed”
Gravatar no longer redirect directly to the default URL you gave, but use an image caching service “http://i2.wp.com/”. This cache insists on your URL ending in “.jpg” or “.png” (there may be others). So I was able to fix this error by supplying a default URL to Gravatar which ended in “.png”. If your default URL doesn’t end in one of those extensions, you’ll get this error.
On a side note, it looks like that CDN is quite open about what images it will serve! For example: http://i2.wp.com/omegadelta.net/wp-content/uploads/2010/11/IMG_9116_500x265.jpg. Did WordPress unintentionally give the world a free image CDN?