Rastaconverter is a Windows/Linux based utility which iteratively converts ordinary jpeg/gif/png files into executable files which display the pictures natively on the Atari 8-bit. Rastaconverter uses all the power of display list interrupts and player missile graphics to allow the maximum number of colours possible on the screen concurrently.
Source and executables https://github.com/ilmenit/RastaConverter
Rastaconverter was/is developed by Jakub "Ilmenit" Debski and is currently in its 7th beta version.
In graphics mode 15, the Atari 8-bit is capable of displaying 4 colours on a 160x192 (typical) screen. This is normally referred to as the limit in the number of colours that this screen can display. However, this ignores colours supplied by "Player Missile Graphics" (Atari sprites) and those introduced by a "display list interrupt". A display list interrupt (DLI) is some code which is executed while the screen is being drawn and swaps out one colour in place of another.