The problem is likely due to a security policy issue in IE: The page is an http URL but some (actually many) avatars are https URLs.
The page of (a random) user - a non-secure http URL
This user has this avatar image (at least at the time I'm writing this) - a secure https URL
On Firefox (and others) the page and the avatar image look fine. On IE - or maybe on SOME IEs, depending on the security settings - the avatar is a broken image because IE won't load the https-source into the non-secure page.
I have the same issue in IE 11, and since I hardly ever use IE my (browser security) settings are the default as Microsoft chose them.
It seems my first guess was wrong - but maybe not entirely, so I'll leave it here.
The real problem seems to be IE Tracking Protection. For some reason downloads form gravatar are blocked, in the console you find message "SEC7114" (link to a Microsoft explanation). When I disable tracking protection in IE the gravatar image loads just fine.
When you go to
C:\Users\[[USER]]\AppData\Local\Microsoft\Internet Explorer\Tracking Protection
you'll find (text) files with the tracking protection data used by IE. In one of those files you'll find a line that
More details about IE Tracking Protection lists can be found here. Use notepad++ or some other editor than standard MS Windows notepad.exe though, those files have Unix line endings!
Here is the Microsoft page where IE users can install tracking lists from. It is likely that it depends on which tracking lists are actually installed and enabled. I have found that gravatar is in at least one list that I installed, the one from abine.com (link to the text list).
I also found this Meta-Stackoverflow issue: "Is Gravatar a privacy risk?".