Matthew, I have worked with FT8 a little bit, and have experience with several programs for making maps of that size you mentioned or larger. The Wiki link posted by Silvyre is perfectly on point. If you want someone to test it with, and to give direct feedback/tips over your Roll20 chat or PM, feel free to send me a message asking to connect. I'm curious why your initial transfer came out blurry and what resolution/format you settle on to improve it greatly. I would personally make a few versions in JPG format of differing resolution and compression amounts, and then test in Roll20 to find the optimal file-size version of your map that still looks good when your players zoom to a level they might use (like say 120%). Once you find the sweet spot, there is really no need for overly high resolutions (that could slow down loading speed and scrolling by a few seconds).
