I see your problem there:
I guess your screenshot was taken in 16:9 resolution, but blizzard only supports 1:1 resolutions with scaling of 2^X.
So you can either scale it down to 1024x1024pixels or up to 2048x2048pixels.
If you did so, you have to go to Edit->Transform->Scale
then you have at the top bar W: 100% and H: 100%
now you just have to make the H: 100% to H: 75%
then apply these changes and make the parts with opacity:0 to black or any other color and you're done.
(if they are invisible and someone uses an 4:3 monitor they have an black void space with everything painting on it each frame like those old error messages when Internet Explorer died).