This photo uses the line of cheese wheels to draw the viewer's eye into the image.
It may not be easy, but it is simple. The three most important things to nail down if you want to get your photos accepted at this, or any, stock site are focus, lighting and composition. Subject matter and creativity go along way towards getting sales, but if these three elements are not in order, your image won't pass the initial screen by the reviewers. If it isn't online, it can't sell. It's that simple. Let's look briefly at each.
Focus-Remember to check focus at 100%. I struggle with this on my little monitor and sometimes I would miss areas of poor focus, so I'll share a tip that I use. After reviewing the entire image, select the areas that are critical to your image. For example, on a photograph of an animal, the eyes are probably a key area. Crop this area and review at 100%. Repeat for all areas that are critical to the image. Personally, I like to use a wide depth of field for stock, capturing as much of the image as I can in good focus. That way, a designer can choose to crop the image for use in a variety of ways. A tripod can be an indispensable tool to avoid camera shake and get that tack sharp focus.
Composition-a stock photo should be pleasing to the eye and organized in such a way that the eye is drawn to the main subject of the photo. Examples of organizing principles include the rule of thirds and the golden ratio. Consider colro, shape, movement, point of view of hte subject, and the use of negative space in your composition. Remember to check the background and foreground elements and make sure they support your composition! My very favorite resource of composition techniques is The Photographer's Eye: Composition and Design for Better Digital Photos
Lighting-It is important to have sufficient light that will allow you to use a combination of a fast enough shutter speed and low enough ISO to avoid digital noise. While you can widen your aperture to allow more light in, there is a trade off. A wider aperture gives you a smaller DOF, which can make a great artistic tool if used well, but also decreases the usable copy portion of your photo (the area in clear focus). As a general rule, avoid extremes such as blown out highlights and deep shadows that cause a loss of detail in the picture. Experienced photographers may break these rules effectively for special effect, but it is important to know and understand them.
I'll post more on each of these later, but for now I am gong to grab my camera and build my portfolio! Good luck to you in your efforts!