GIMP issue

[The chaser]

-Open your image
-Then, select by color (Majus+O)
-Now, select the part you want to be the alpha part
-Go to Colors<Color to alpha
-Say "Ok"
-Now all that part should be covered by grey squares, which means it's alpha

Unless other programs, you must specify what part of the image is the alpha, so that's why it appeared as black.

I hope that fixed your problem

PD: I would have resized the image to 1024X1024, I find it more appropiate.