What I've done, as admin, to not have certain pics that are not related to me that I upload under my name, is to create another username (For example, I use 'Photo Arcvives" as a username)
Then when I upload these pics I change the username during upload to Photo Archives, or to a current member's username that they represent.
One of our galleries is for pics of artist's works, another is of plant collections of well known collectors in our field.
If they have not registered at our site I might contact them, inform them of 'their" photos we have in the gallery under username Photo Archives, invite them to register, and upon registering move pics to their username.
As I am usually the photographer who took the pics, I generally add my credits in photo description commentary.
I don't do this for all pics I've uploaded. sometimes they stay in my username, sometimes in Photo Archives, sometimes changed to another username.
It's done on a case by case basis and the above serves as a means.
BTW There is no way I know to mass change the username in a group of pics to another username. I do this by editing each pic.
Hope this helps