I love doing this myself with short 1-2k word scenes to help develop the character and my writing at the same time. When writing a long piece in chronological order it's easy to get stuck at a section and the continual revising is tedious when you just want to write. So just skip over to a part that you want to explore and you'll often find it's much easier to get started. Later on when you actually write that section of your novel you can go back to your old short story and use that scene as inspiration.
As the previous commenter mentioned writing these short stories also gives you a chance to practice your craft in a more controlled setting. I find myself writing much quicker when I'm not constrained by the subtleties and pacing required for a full novel, and you can adapt the story to match your writing rather than the other way around.
TIFF is a classic storage format, but PNG is common for web images and isn't going away either. DNG is for RAW sensor output from professional cameras and is not used for edited and published images. However if you're archiving your photo collection or something than keep the DNGs!