Is downloading pictures from a university computer without the professor's permission considered theft, especially if the pictures are of veiled girls taken at a university exhibition?

Your action involved two forbidden things: first, downloading pictures without the permission of the professor or the responsible authority, and second, downloading pictures of women. It is not permissible for you to take or look at them, even if they are veiled, because Allah Almighty says: "Tell the believing men to reduce [some] of their vision and guard their private parts. That is purer for them. Indeed, Allah is Acquainted with what they do." You must delete these pictures, seek Allah's forgiveness, and repent.