Google Play Store: You need fix your high-res icon. It is not a valid image

As described on Google Play Store developer console, icon image should be:
512 x 512, 32-bit PNG (with alpha)

Your picture is (I checked with "file" Linux command)
PNG image data, 512 x 512, 8-bit colormap, non-interlaced

This happened because your image uses only a few colors and gimp decided to use png format with indexed colors so image file is as small as possible.