What is the ruling on praying in the university prayer room (musalla) that contains photographs illustrating how to pray and perform ablution, knowing that the faces in these pictures have been erased? And do angels enter it to encompass gatherings of remembrance (dhikr)?

Photographic images are a matter of scholarly disagreement, and the more preponderant opinion is that they are permissible out of necessity. They do not prevent the entry of angels. As for hand-drawn images, they are not permissible and prevent the entry of angels, unless their heads are removed. It is permissible to hang permissible images, but they should not be in front of the worshipper so as not to distract them.