When I ran groups...I did not like all the many separate "Thank You" discussions started....especially when I specifically asked people NOT to do that...
I personally just post a "thank you so and so for featuring this picture on such and such a group" on my specific pictures page
Also, JMHO, I think it's a bit "tacky" when people post in their descriptions of the picture a "this was featured in x,y&z groups"...but that is just my opinion. Also, some people changed the actual NAME of their picture to include where it was featured! Bad idea as they break all the links that were already shared while it had a different name.