Hello and thank you for this wonderful plugin. I have been using this for some time on my sites and recently tested the compatibility with the RICG Responsive Images plugin. The responsive images feautures of this plugin are going to be rolled out in wordpress starting from version 4.4, more about this here. There are at least two implications. From what I observed, the plugin continues to work fine, with exception that the srcset attributes are populated with gravatar.com info, for example:
the srcset part throws a Google pagespeed warning and I assume this code is there to support retina images. I don’t believe there is a valid case to have gravatars responsive on any device.
The second issue is that the plugin creates the cached files copied from gravatars site without any extension. While this is OK for the purpose of displaying, it defeats the image compression options, via ImageMagick, as it does not recognize images for files without a valid image extension.
Your feedback shall be most welcome.