TVPaint 3.6 is probably the best paint program for RTG. It doesn't work at all in native Amiga modes. It's freeware. Here's a quickly googled link for a download:
http://www.amigafuture.de/downloads.php?view=detail&df_id=3598 For image processing ImageFX. 4.5 is the last version but any version from 2.6 on will be good. Some paint features but a lot more image processing. Not freeware unfortunately.
Deluxe paint will work in 8 bit RTG modes with Cybergraphics but not Picasso96. The best version is 5.2. 5.0 has a bug in the airbrush.
Photogenics is really unique in the way it works. I don't know of any Paint/Image program that works the way it does.
Art Department Professional also works in RTG modes.