"nospam" wrote

| bmp is very, very rarely used anymore.

Of course not, on Macs. The OP seems to
be on Windows. A BMP is the most basic
bitmap format. A grid of pixel values. It's
what *all* raster graphics are. JPG, PNG,
GIF, etc all render as bitmaps. Only the
storage varies. A BMP file is only that grid
of pixel values, with a very minimal file header
of something like 22 bytes. The image you work
on in Photoshop is a bitmap. Most of the filters
you can use in Photoshop are essentially math
formulas applied to that bitmap.

So on Windows, the only sensible way to work
with non-RAW is as BMP, or as TIF if one
prefers it compressed. But in that case a TIF
is only a compressed BMP. One could work with
PNGs, but there's not much point. If one works
in JPG then each save is lossy. If one works in
GIF then the number of colors has been severely
limited. There are different ways to go to 8-bit
color from 24-bit. Getting the best GIF can
sometimes take a few tries. So there's no sense
reduing from BMP until editing is done and one
wants a finished copy as GIF.