Fact: There are tons of (Yahoo-owned) Flickr users.
I’ve always been intrigued by affiliate marketing — which basically allows you to promote something, then get paid if someone buys that product/service as a result of your promotion. I think there’s an opportunity to join affiliate marketing campaigns for all the photo printing companies (Shutterfly, Walgreens, Wal-Mart, Ofoto, etc) and by using the Flickr API, you could create some Windows/Mac software (using Apollo?) that allows people to do some quick edits to their photos — and easily re/upload them (a la Google’s Picasa software). Then allow people to easily get their photos printed via the photo printing services you have affiliate marketing campaigns with — this will allow you to earn income from the users of your software (that you give away for free). If your software is good enough, then tons of Flickr users will come and use your software, and hopefully easily get their photos printed via your software.
P.S. Here’s another cool way to earn money via affiliate marketing with Flickr — upload photos of products, or take photos of products, then link them to an affiliate marketing link. Example, upload a photo of an Apple TV and link it to somewhere that the user could buy it — such as Amazon.com, but using your affiliate account with Amazon, so that if someone buys the product, you earn money!