FYI
When dealing with PNG in Photoshop it says 24 bit PNG with a checkbox to include transparency (alpha, extra 8 bits which makes it 32). This can be confusing. A 24 bit PNG has no alpha. If you include transparency using a "check box" in a paint program... it becomes 32 bit but the software doesn't bother telling you that.
They like to keep it simple and refer to only the color portion of the image. So it is 24 with or without an alpha...
... unless you are saving out targa in Photoshop... <sigh> then they get it right. They offer 16, 24 and 32 as the save options for targa. 16 is grayscale, 24 would be RGB color without an alpha and 32 is 24 bit color with an alpha.
They should be consistent but... I can understand why they do this... maybe it is the proper way... I have no idea.
Don't even get me started on 16 bit per channel color images (48 bit). I have to upgrade photoshop to work with those. Apparently PNG has support for that format! Coool!
p.s. An 8 bit PNG is an indexed color palette, 256 colors max... the transparency for that is just one color out of the 256 available with the 8 bits. So transparency is "all or none".
-vern