Converting 8 bit PNG to 32- bit with alpha
Fixed it using https://pixlr.com/editor/. Just open and save the image with full quality.
With imagemagick:
convert <file> png32:<newfilename>.png
With Gimp: open, add alpha channel (Layer > Transparency > Add alpha channel), set RGB mode (Image > Mode > RGB) and export as png.
Gimp -> export as -> png -> choose 16bpc RGBA