Thanks.... Looks good..
There are loads of funky file formats for the c64 (lots of new ones that use hardware tricks and are way way over my head, I only understand the native ones) - But I recon if your program could just spit out a BMP downsized to a user selectable size and dithered pallet (maybe 4 colours, 8 and 16 or custom maybe by importing a .csv of the pallet in RGB Values ?) , then that would be a really really good head start for all sorts of cros devs.....the output could then be processed in other tools....
Things get complicated with C64 screens, for hires eg... 320 x 200 Its basically a B+W image, BUT each 8x8 section can have a different colour for the foreground (1000 characters and it used 1K for this colour data)....
Now, once you go up to Multicolour, you have only 160x200, but each 8x8 (well 4x8 due to using 2 bits per pixel) , can have a pallet of 4 colours.... 00, 01, 10 , 11 - and 2k is allocated for the colours....
FLI's and stuff.... I don't know... But I presume that the images are pretty big, as they are kind of dithered every other frame, and the image (2 pixels per pixel) is also shifted by 1 pixel, to improve the horizonal res... Yes... complicated..... They also use overlayed multiplexed sprites, to give more colours and stuff..... So.... you could really loose sleep !!!
I also guess, a lot of just reducing colours and resizing could be done in lots of other pc based tools like Gimp or whatever.... but like my mate says... you seem to have a really good handle on the dither algo's.....
Hence why I suggest just spittting out a very nicely processed BMP image.... and let the 'other platform' guys worry about it

Keep up the good work....
Kind Regards, Colin