Nice challenge, I got stuck for a few hours after getting the original file out thinking there was no obvious indicator where to go and what a password may be.
If I missed something “in the corners” can someone PM Me to say what 
I tried the usual ways in Gimp as well to try and reveal data and saw nothing so I assumed data was hidden another way, and I was blindly trying “obvious” passwords and one worked to give me the final stage to work on.