Pictures are a good way of showing the PROGRESSION of your relationship over time as well as INTEGRATION into each other's lives and that your relationship is known to the PUBLIC (meaning your circles of family and friends). Think about those things when choosing pictures. It's pointless to send a picture of your spouse with his/her family unless you are in it too! Now submitting a picture of your spouse with YOUR family is a good picture and vice versa.
Remember that it is the culmination of proof that the VOs consider. Pictures themselves won't make or break you and there are no hard and fast rules. Here's a couple of points to keep in mind:
1. I want to show how long we've been together
2. I want to show that we have met each other's family
3. I want to show that we've done various things TOGETHER during our relationship (travelled, went to events, stayed home, celebrated birthdays & holidays, played sports, etc.)
Keeping these things in mind will help you sort through what you are going to submit. Do the best you can with what you have.