Try using Krita.
it's free
open the .png file
then on the right side of the window, right click on layer one
go down to split alpha
Alpha into mask
you will have two files
uncheck the black or white one
select the other one and you will see the correct image
PNG files have a mask layer, this easily removes it.
edit to desire then save
PS. make sure to use the correct color or you will have invisible holes in the terrain.
you can then CTRL A CTRL C and paste it into photoshop if you want to further
enhance it.