Indeed. And from what I've heard, iStockPhoto has a ridiculous process that you have to go through to get approved...I think they are basically so "full" of submitters, if you aren't already in the system it's hard to break in. Maybe focus on that? (Not saying offer terrible pictures from just anyone, just saying that maybe their site has gotten almost too big...maybe there is a glut in the market.)

Something else you might consider doing is good integration/importing. For example Flickr has a check-box where you can easily "offer your photo for sale on Getty images"...while you probably can't get Flickr to add a box for your site right away, use their API to allow new users to easily import some or all of their Flickr photos right into your site for sale.

Perhaps also consider offering a mix of high-quality free and paid photos. For example (again using the Flickr API since I am familiar with it to some degree) there are a lot of amazing photos on Flickr that are license Creative Commons, meaning they are free for use in non-commercial works. Maybe go through yourself and find a few hundred really high-quality ones and offer those alongside the paid ones on your site, categorized and tagged.

There are definitely opportunities to be had, it's just a question of finding them.