With this particular exchange (there are a few different ones, this is the "blind print exchange") you can only send prints made with traditional processes, like developing film, and printing "the old fashioned way" with an enlarger and print paper and chemicals to develop the film.
Originally Posted by analoguey
There is ONE exception, and that is if you use Transparency film AKA slide film AKA E-6 processed film. The reason is that the kind of paper and chemicals that you need to print these with an enlarger are no longer available unless you have a stash hidden away, so ONLY with slide film, you can scan them and then have a lab print them. But labs these days have two main options on how they print, they can use their new printers which (I'm making this basic guys, I know this isn't exact) will print a photo just like a home printer can print a photo with ink onto photo paper, there are no chemicals or developing involved. The OTHER way to print is a fancy machine that MIMICS an enlarger, and the image is then "enlarged" onto print paper in a dark room and developed in chemicals just like the "old fashioned way". So for this exchange they ONLY allow CHEMICAL DEVELOPING for the printing part, so you have to call your local printer and make sure they are printing with chemicals, and not printing with ink.
Does that sort of make sense?
This is all in the FAQ, but if you're not familiar with the terms they use "light jet printer" you won't know what they mean so that's why I'm attempting to spell it out for you.