I read these rules, and I read it as they don't care what the source is, "Image data files created with any device capable of taking still images [including digital cameras]" - it doesn't exclude non-digital cameras. But you have to submit the image in a digital form, so you have to scan the negative, slide, or you could presumably scan a print. Technically, this means they don't accept images on film, as in you can't mail them a slide. I can hardly blame them for that. They're probably not set up to mail your slide to the judges, either.