It seems to be impossible to change a Retool user's profile photo. This isn't very important when logging in with Google, as it uses the Google account photo. On the other hand, users registering in the "classic" way cannot add a profile photo.
You are correct that it's currently not possible to change a non-Google profile photo. I wish I had better news for you here, but if you're not signing in with Google SSO (to use the image from Google), the only way to set that would be with On-Prem Retool with manual updates to the
I added this thread to our internal request ticket and will keep you updated!