I mean they are a part of graphics, but if there is png, there should also be Bitmap.
We don't really have a lot of tags for image file formats in use.
png is really the only one, and it only has eight questions, and glancing through those questions, the
png tag is either noise or not all that useful. So, I don't really see the need for the tag you are proposing (I'm not even sure
png is worth keeping around, honestly).
That said, because you are talking about a file format and not 'bitmaps in general' I would suggest that you use the tag
bmp for this, if you were to tag it at all.