Changing user's profile picture source from internal to external and/or backwards, some browsers cache the prior picture file instead of showing the other one (cf. Issue
external -> internal
Description: Cache problem appears often, if the prior redirect to the external source answered code 302 which is sometimes misinterpreted as code 303, though RFC, however.
Current solution: Answering code 307, "temporary redirect", seems to cause most browsers to load the internal picture correctly.
internal -> external
Description: Cache problem appears rarely, however. For instance, my Opera shows the old, internal picture -- although it requested the Gravatar picture!
Potential reason: Opera's http request to Gravatar contains a "If-modified-since"-tag, such as Gravatar's response is code 304 ("not modified"). Maybe this leads Opera to use the (cached) prior, internal picture file.
Proposed solution: I have no idea how to solve this by now, unfortunately.