It's fairly common for net images to use progressive, so you will often at the very least have to re-encode them. I'd also check their size to try to normalize it to something convenient for q3 offset/scaling. Try move ALL of your textures out of your texture folder and see if it still crashes. If one of your textures is the problem, then you will get a lot of missing textures, but it won't crash. If it still crashes, it could be something else.