For our common-law application we included around 20-25 photos... all the pics were printed via a Word document, a couple photos per page and a caption explaining what they were and the date taken. We ensured the pics showed dates from the very beginning of our relationship right up to the point we submitted the app. They all involved us hanging out with my friends, her friends, my family, her family, us taking vacation together, and a few of just us celebrating important relationship events (like anniversary, getting our puppy, getting engaged and showing the ring etc).
The most important pictures will be the ones with family and friends... since if you were in a fraudulent relationship you would most likely not want your fake partner to meet and get acquainted with them, and would try to keep the whole thing private.