The serious stock agencies is where you can make serious money. Even though I haven't contributed anything in years to the two that sell my work, I still make a very nice chunk of money every year.
But it is seriously hard to get in.
1/ As tharmsen said they will not accept any work that is not 100% perfect technically, according to pro standards.
2/ When I got in (and I don't imagine it is any different today) they required a few hundred photos on the same theme/subject before they would even look at your work.
3/ trying to sell what you shoot for yourself is not the best way to approach this business. You'll do better if you think of what types of photos have the potential for multiple sales and shoot those.
Couple of examples: Most of my stock income comes from a set of over 4,000 photos of dogs. No artsy stuff in there but you will find everything else, from cutesy to... They sell to calendar companies, dog book publishers, dog magazines, encyclopedias, etc, etc.
A friend of mine whose entire income comes from stock has been photographing animals for years. One of his best selling photo is a very simple image of the common box turtle on a piece of lawn. Just a turtle and some grass but it has sold hundreds of times.
4/ Releases are a major concern. I imagine that graffiti would be considered works of art by the agencies and without a release from the artist of each piece, I don't think you stand a chance. I would keep that in mind even if you try to sell them any other way. The artist owns the copyright to his works...