Image sourcing is so important and so many people don't do it the right way, and I'm glad you enforce it as part of the approval process. I try to get around the problem entirely and take it as an opportunity to enrich my blog posts by showing my own photos. Even if they're not totally related to whatever I'm posting I try to find something that has at least a similar mood or theme. But that's just a personal preference and I know it doesn't work for every genre.