I have personal experience with the photo-stock site Fotolia; and out of 112 submitted images, I have had forty-three accepted. Of those forty-three, thirty-four have sold, some many times over; my top seller has sold 126 times. Some contributors have thousands of images with Fotolia. (For a link to Fotolia, see my Hubpage’s ‘Profile Page’)
Do not expect to be able to draw from your existing images and have them accepted. Have a look through their gallery, see what sells, look at number of ‘views’ compared to number of ‘downloads’. Even in the 'Best Sellers' gallery there are some images of relatively simple subjects. Read the forums and see what their contributors have to say.
To get started, look for an image that you feel you could achieve, think of a new angle on it, and take your shot. Fotolia, images need to be a minimum of 3.7MP, pin sharp focus, and no noise. To avoid being disillusioned by repeated rejections, you need to be very critical with your own work. Look at the free images; these are pictures that have not made the grade, but the photographer has allowed to be downloaded for free (though why anyone would agree to this is beyond me), this will give you an idea what not to submit. Do not worry about cropping; the purchaser of your image will do that to suit his or her requirements. I may add a bit of colour saturation and contrast to my images, but not too much as you do not want to introduce noise in the image (simply put, noise is a degradation of the image).
There are numerous other photo-stock sites that you can submit your photographs to, and which one is best will come down to your own personal preference. To compare sites, select similar photographs from a number of categories from each site and look at how many times they have been viewed, and how many times they have been downloaded (purchased). They also had different rules about what you can submit; for example, Fotolia do not except images containing brand names and logos, images of artwork and designs that have a copyright such as vehicles & patented household items; or photographs of buildings and people without a signed model release. Once you learn the rules though this is not a problem, you just learn to avoid certain subjects.
You can submit the same image to a number of photo-stock sites but they pay better for exclusivity. It’s a great feeling the first time you see one of your photographs in a publication, or used to illustrate a website, so have a go, and don't give up if at first you receive a rejection.