In article , Carlos E.R.
wrote:

I'm unsure a jpeg can be considered a format usable for bitmap
manipulation, because it is a lossy format. So someone creating or
changing a bitmap would not use jpeg because individual pixels can
change or just be lost. A bunch of spatially contiguous pixels can be
grouped and get the same value - when in the original they were different.

of course it's usable. it's done every day.

it's obviously preferable to use raw, but not required.