When trying to use an .xpm image with transparency that transparency will replace pixels despite being completely transparent. Using the mac mlx transparency works fine:

Mac left, Linux right, using the same test code where first blue.xpm is put to the window and then red.xpm is put to the window. (both 8x8 pixels, hence the weirdly thin top bars of the windows)
When trying to use an .xpm image with transparency that transparency will replace pixels despite being completely transparent. Using the mac mlx transparency works fine:

Mac left, Linux right, using the same test code where first blue.xpm is put to the window and then red.xpm is put to the window. (both 8x8 pixels, hence the weirdly thin top bars of the windows)