The beauty of web design is that it is all in "the numbers". As long as you follow the most basic principles and stay within the suggested numbers that will serve to measure the size and amount of all aesthetic and informative elements in a webpage, you should not have any problems.
The first tenet is that each graphic must serve a purpose. The least amount of distractors and unneccesary pictures the more effective the search and usability of the page will be. Hence, avoid creating graphics just for the sake of decoration. Put the decoration to work.
Second, avoid repetitive animated images. Research shows that some computer users might be overstimulated by specific repetitions of graphics and may have seizures as a result of it. Avoid using and adding those.
Color is not meant to have any coded significance. Use color as much as possible and do not compromise assigning emotions, meanings, or goals to specific colors. What would happen with another graphic using the same colors? Is the user meant to add meaning or dissect and analyze its potential significance?
Use the ALT component and always offer an alternative definition or description of the graphic. This is done precisely to avoid that any viewer assigns the wrong meaning to the graphic, or that a user misinterprets the graphic or what it is doing on the page.
Graphics should be compatible with all resolutions, browsers, and browsing speed. A graphic should be made to fit every search and should pop up or be easily downloadable. Anything that takes away from research time will be automatically rejected by the user.