As the producer of a commercial app can I ask users for media to use in the app in exchange for small payment (i.e. $5 for each pic used) to fill in content of the app? If so can I have users email me photos that I can use or should I have them go through some submission form where they agree to some terms?
You should have the individuals submitting content agree to terms and conditions that give you the rights you need to use the content, modify it, display it, etc. (including your ability to grant rights to end users of the content).
The reason: To minimize the likelihood that individuals submitting content will have any reason to complain - or even bring a lawsuit - because of how you used the content.
Disclaimer: This post does not constitute legal advice and does not establish an attorney-client relationship.