The tiles need to be converted.
They contain a comment.
Belmap for Palm OS 5 can’t read PNGs which contain comments.
They contain more details like the gamma.
Here you see the start of a PNG picture with header tag (IHDR), color space (sRGB), gamma (gAMA), resolution (pHYs) and pixels (IDAT).
My program CleanPNG can do this conversion.
© 2017 Cliff Huylebroeck