Apple (and possibly Windows' GDI) use a non-standard pixel format internally, BGRA. This means that all image buffers that come out of CGDisplayCreateImage / CGImageGetDataProvider / CGImageGetDataProvider / Apple's graphics APIs have the B and R pixel swapped. It would be very convenient to be able to tell image that a raw buffer is BGRA, rather than having to swap them manually. The information could also be stored, and used to avoid more byte-swapping, in case the Image is saved to a format that also uses BGR.

Edit: this format is commonly called ARGB.
Edit 2: confusingly, OpenGL refers to this as BGRA. In any case, we should support it.