Many years ago when I was freelancing, whilst studying for my photographic qualifications, I had a reasonable income from agency stock work both in the U.S. and the U.K.. One of the U.K. agencies turned out to be a bit crooked in the end, selling work for major use i.e. advertising, major international exhibition, but not paying. So you have to beware. Most photographers go wrong in thinking that agencies want photos of pretty views or dramatic landscape but, they are overstocked with these. Much of my income came from simple things, a postman emptying the post box, piles of litter in the streets or floating in the river, fishing boats landing their catch. The greatest demand is not for a stand alone picture but one that will illustrate an article. In the end editors were actually asking for my work. SO, think outside the box and keep at it, but avoid the chocolate box type image if you want to see your work in print. See,
www.axisweb.org/artist/6196 for sometimes it is the caption that makes the work.