Free content is always nice, BUT to make yourself stand out above the crowd, make it HIGH QUALITY. Some places do give out nice stuff, but some places give out low quality stuff that I'd never use. The higher quality, the better
http://www.oliver-klozov.com is an example of a place that gives out GREAT content to it's webmasters. It's the best free content that I've seen a sponsor give to it's webmasters.
OR for those people that want the lower grade "web ready" pics, maybe you could have 2 versions of each pic, a "web ready" pic and a raw pic, so they could choose.
I can't think of any other things that you could offer. The checks every 2 weeks is also a good thing. You already hit up the other things like payouts on all exits.