Each of these works with my PNG viewer, the alpha is at palmboxer.sourceforge.net as Bxr019xbin.zip and Boxer019x.zip. It works with any 8 bit or less grayscale, or CMAP (a palette of byte sized RGB entries) PNG file, and supports all line filter modes. It can't handle truecolor or 16 bit greyscale but that is well above the palm (you can still box and transfer, just not display). Unlike other viewers I don't require cropping or decimating the image to fit. But it may take several seconds to unpack and start (the smaller files will open very quickly).
I approximate greys on the noncolor palms, but the color palm will display these in color. The displayable size is limited only by free memory, e.g. you need a meg free when running the program to display the raytraced PNG view below - only tested on POSEr (the emulator) since I don't have a real IIIc). Most are from http://www.geocities.com/SiliconValley/Lakes/6686/test-images/ Most of the rest are from the PNG homepage.
pengbrew.png (yes, it looks wrong here since it handles transparency)