Including content that is not pertinent to your goals or the topic of the site is the biggest mistake you can make. The reason being, at first, a number of your visitors will come from search engines . A site that has a lot of off topic information wastes the visitors time because they have to bore down to the content that they came looking for. Content can be divided into three types. These are aesthetics, navigation and functionality.
Images are for aesthetics and quite often navigation.
The overuse of images is one of the biggest mistakes I see on most sites. A number of developers come from a graphic design background and so they use a lot of images because they are used to typographic layout where images don't really affect the use of the information. This is a problem where the internet is concerned because of bandwidth and hardware constraints. I don't know about you, but I frequently leave a site because the download times just don't warrant the time spent getting the information. The biggest culprits here are Java and overuse of images.