The transparent part of the image contains a lot of random-looking colors.

To see them, open the image in Paint.

Get the Red, Green, Blue values of the pixels and analyze the numbers. They are not random.

A lot of the pixels have the same value of one color--focus on them.

Take those pixels and find the other color that has values from 1 to 26.

Translate the numbers 1 to 26 to letters A-Z

Apply a Caesar Cipher to the letters, moving them forward in the alphabet.