When ImageFileFormat::loadFrom() is called on a PNG binary resource, the result of isRGB() can be different between Mac and Windows, even though the binary resource is identical.
Well, on the mac it uses CoreGraphics to load the image, and I left a comment in the code saying that it doesn’t work with 24-bit images. Can’t remember exactly why though.
I'm seeing a strange problem on my Mac as well. I'm using an 8bit PNG (small 2x32) and pixels that I know are (255,0,0) are returning things like (255,38,0) on a getPixelAt() call. Not sure if some bitmask is off somewhere or what.