Resolver II just published this in the parallel thread - http://community.powerbi.com/t5/Service/app-workspace-icon/td-p/220627
I had an interesting situation. My team/group icons were not showing in Power BI Service Workspace list. Premier support said they had fixed something (didn't say what) and it was working again. They asked me to update an icon and it correctly showed in the workspace list, but I had to redeploy the APP for it to show on the APP's tile.
Subsequently, I added a photo to a group that did not have one before, and it showed up straight away. I did not like the photo so I deleted it from the group, and now none of the icons show up on my Power BI Workspaces list! I added the photo back in again, and all icons reappear. Deleted the photo and all icons disappear. Tried it with a second group icon and the same happens.
If a team or group never had an icon in the first place then it is ok, but if any team or group has had its icon deleted, then no icons show in the Power BI Workspaces list.
Try adding a simple icon to each team/group.