This is now supported as of commit 2fd21d6d483695a658cc3baf545020700f70e8db
The beeper apps have different levels of support for contact photos (don't shoot the messenger).
Android: photo will display correctly, but chat name will end with "& iMessage"
MacOS: photo will display next to each message and in the participants list, but not the overall chat. However, chat names do not end with "& iMessage"
Using the bluebubbles android client, contact images come thru, but using the beeper integration they do not.
beeper:
bluebubbles on android:![image](https://github.com/mautrix/imessage/assets/684920/e3e770f3-9ccf-46a4-a984-a655127b03f5)