I found a very simple solution to the problem.
One can use a free program called CodeTwo Active Directory Photos to set the profile images using a GUI.
You must use the program from the admin account on a local computer already attached to the domain (admin@domain.tld). It will automatically connect you to the domain since you have permissions, and allow you to set your profile images for each account.
SoGo doesn’t use the images, but you can enable gravatar support in the program to accomplish much the same.