How do I convert an image to 24 bit?

There's no such thing as a 24 bit depth image really in general usage. Each channel is 8 bits so any RGB image that's 8 bit would be... Red 8 bits + Green 8 bits + Blue 8 bits = 24 bits.

So if you're image is at 16 bit RGB say a raw file, just convert it to 8 bit RGB.