Read the rules thread. I will not repeat this.
I added a relaxed upper limit of 800px wide for any images due to the increasing prevalence of widescreen monitors; if yours is bigger and you do not enclose your image in spoiler tags, you will be infracted. The 640x480 limit has been in the rules for over a year. Even then, use your judgment; make it smaller than 640px if at all possible. Either shrink your image down to 640x480 or put it in spoilers. This isn't rocket science.