The most important thing I have learned from submitting images for the past few years is probably the most simple tip I can pass on to anyone. I never upload an image the same day that I create it. Whether it is a photograph or an illustration, I always wait until the next day and take another look at it with "Fresh eyes" You will be surprised at the tiny details you miss after working on an image for any length of time. So, wait, zoom to 100%, inspect, then upload. Like many of you, I have had an image rejected and discovered that the reviewer was exactly right, there was a bit of purple fringing or a pixelated line somewhere. In the long run, it really is a time saver. Get it right the first time and you won't need to resubmit the image after that "Duh" moment when you realize, "I should have seen that!"
Photo credits: Emin Ozkan, Feverpitched.