I’m no developer, just checking in to ask: are you aware of PicToBrick? http://pictobrick.de/en/pictobrick.shtml
Several LUG members have used this (and I believe at least one other site / software package) to generate a parts/colors list for LEGO mosaics. It’s fairly useful, although I’ve found that there’s always some refinement necessary with the human eye while actually building, to get things to look right.
Could be a fun challenge for you to write even though some similar software exists – or to find aspects to improve upon what’s out there!