Best Lossless Hi-Res Photo Storage with PHP

I'm working on a photo site and most of it is working. The intent is to store original high-resolution photos that will only be accessed when purchased, and then a variety of thumbnails that will be accessed when viewing. Because they're high-resolution, I need maximal image compression but I can't sacrifice image quality. With hundreds (or more) of many megapixel images, space requirements quickly soar. I am currently using PNG, but that's not all that great for lossless photo compression. JPEG 2000 is significantly better in terms of file size, but I haven't ever used that with PHP (and it seems I'll have to use it via ImageMagick or something). Are there any other (free) formats for high image compression out there that I can use (maybe even just via the shell) with PHP?