Unless scheme="https" is set in server.xml, gravatar images are served over http, breaking https.
However, if scheme="https" is set, then the application link between JIRA and Bitbucket is broken on the JIRA side.
When this happens, the following message will be displayed on the JIRA side, even though the Bitbucket side says that they are connected.
"Application 'Bitbucket' seems to be offline. Click here to Relocate."