You should let the user upload what they want, but just make it clear in your Terms and Conditions what the requirements for a photo are. Filters, cropping, etc should be fine and the photo will still be original.
You should put in place a moderation tool so you can easily delete / hide photos that don't comply with the Terms (i.e. in an admin section), and allow users to 'report' images. It's extra work but it means you have overall control of the photos that user's upload.