I signed up for Fotolia a couple of years ago. Pricing varies, but I normally make around 25c per photo. They pay out when your account reaches $50, which is therefore 200 downloads. I'm about half way there and still haven't received a payment. The photos I like personally don't sell because there is no commercial value. Stock photography requires very specific types of photo and there are tens of thousands of highly accomplished photographers (and illustrators) selling their work who you have to compete with.
Over 90% of the sales I have made have been of one particular photo. It seems to be a photo that people want and that other photographers don't have to offer. It was just a quick snap I took while wandering around a Burmese refugee camp on the Thai side of the Thai/Burmese border in Mae Sot. The guy who recommended Fotolia told me that his best selling photo was of garbage collectors collecting garbage. Photos you think will sell don't, and often the ones that sell are the ones you don't think will sell.
People don't necessarily want aesthetically pleasing photos, they want photos to illustrate a certain subject. There are also lots of cliche photos, for example, happy families in perfect situations, attractive businessmen and women shaking hands on business deals, etc. These contrived photos are not at all like the real world photos I take.
Then there is the business of model releases if your photos contain people, being careful not to include trademarks, submitting noise-free images, etc. It can be a real pain meeting all the requirements and submitting the type of photos that are wanted. I have had far more photos rejected than accepted.
When I started I thought it would be an easy way to make money, but that isn't how it has turned out. It's still quite satisfying to think that someone has actually paid money - even if only a small amount - for a photo that I took, but like most things online these days the only people who make any serious money are the big boys.
You mentioned Flickr in your original post and I thought that images could now be submitted to Getty Images via Flickr.