I keep getting "can't upload image" but no explanation of why?

Hi, profile picture? Could be a policy preventing you from changing the image. Check with your IT admin if they are controlling this with the -SetPhotoEnabled parameter.

Check this conversation as well https://techcommunity.microsoft.com/t5/microsoft-teams/quot-there-was-a-problem-saving-the-photo-please-try-again-quot/m-p/808222