If you have a big enough following online then a site like 500px or deviantart or wordpress can make it quite easy to do. If you don't, stock photography is your best chance to "sell" images but then that's not really selling work so much as the time taken getting the images. The best way still remains getting clients in terms of actually going pro, or if your images are strong enough and your work recognised enough, gallery representation is a good way as well and adds legitimacy to asking real prices for your work i.e. over £300 per fine art print, increasing with size, with limited runs. There's no value to mass producing photography.
20x200 also seems pretty cool if you've got the standard of work to get in the next time they accept new artist submissions. Depends on the standard of your work of course.
The main point when it comes to selling photography is that you have to be 1) astoundingly good and 2) have a reputation to sell prints (rather than commissions) for 99% of photography. Of course stock photography is pretty accessible to anyone with a decent lens and a DSLR but it's not really a genre that many people that find that interesting and certainly won't end up helping sell prints and the like.
