1: It makes .png right away, and even if it didn't I don't see what difference would it make. Format .png is a raster format(bitmap) itself, very similar to .bmp, the only difference is that is can be compressed, but that doesn't erase any usable data in it.
2: Each color takes 1 byte = 8 bits, one byte for each channel, R(ed), G(reen) and B(lue). That sums up into 3 x 8 = 24 bits(not bytes). You can also add one more channel for transparency, usually called Alpha, which would be the 4th byte and then one pixel will have 32 bits.