Gray Tree Frog

They aren't "difficult" to tell apart, from a picture they are impossible to tell apart.They only way to tell a Gray Tree Frog from a Cope's Gray Tree Frog is by counting the number of chromosomes, or by listening to the calls. The calls are just barely different, and can only be separated from each other if the frogs are at the same exact temperature, which you have no way of knowing.

or Cope's Grey which are difficult to tell apart. http://www.dnr.state.md.us/wildlife/Plants_Wildlife/herps/Anura/GrayTreeFrog.asp