I have always been under the impression that for users that login in to Yammer for the first time their profile picture is pulled from Exchange Online, just like with the O365 suite bar. Now, we don´t use EXO but still some (not all) of our users get a profile picture and I have no idea what mechanism is resonsible for that.
Firstly, I though it might be pulled from the mySite host, just like with Sharepoint and OneDrive for Business profile pictures. But Creating a new user in local AD, synching him to AzureAD, adding his profile picture to the mySite host still leaves him without a Yammer profile picture. SPO and ODFB received theirs.
Secondly, I suspected it could have been the AD user attribute 'thumbnailPhoto'. I added a picture to the local AD profile, re-syched to Azure AD but to no avail.
Thirdly, I was thinking of a new user having had assigned an EXO license within your E3 plan upon creation. Did that to my test user but this didn't change anything as well.
So, how do some users get their profile pictures in Yammer if they didn´t upload them themselves?