Why?
@aparrish I use ProMotion in tileset mode, export the graphics as indexed 256 colour .bmp files (it was easier to write a loader), and everything else is just quick and dirty commandline-driven conversion tools written in C++ to output converted tiles, maps and palettes.
I'm more of a coder though - I'm fine coding tools, but I couldn't draw a decent sprite if my life depended on it.
@aparrish At the moment I have a script that says where in the source image an item is, and then after conversion I write out an assembly file with the tile numbers.
One of these days I'll write something better. The more I write this down the worse my solution sounds in my head...