Windows 7: Difference between BMP and PNG? (Start Orbs)

I was about to make some custom Start Orbs. I was wandering what was the difference between BMP and PNG when it came to Start Orbs. Would it even matter which one you used? Because I usually use the PNG file when choosing my Start Orb. I don't see a difference when using a BMP file though.



Another question: I get my Start Orbs from DeviantART. I was wandering, why do people include so many Start Orb PNG and BMP files. For an example in my picture. Is there a diffidence of them?

PNG files contain transparent layer i.e. the white color becomes invisible where on the other hand BMP images are uncompressed and lossless quality but they do not contain transparency and are like raw images. Quality of PNG files are also great.