Visitors come to your site for different reasons. An academic visitor whose purpose is research may be willing to tolerate long passages of uninterrupted text. The recreational visitor most often will refuse the challenge of wading through blocks of text and look for an alternative site on the same subject that offers a visual as well as literary approach to a subject. Images provide the opportunity to pause and reflect on the text that accompanies them and often fills in the blanks left by words, which may or may not have the meaning or clarity you intended. They create the environment of your site and provide unity and continuity as visitors move through it.
Images perform several functions on a web page.
As with anything, advantages are countered by disadvantages. Pages with images take longer to download. The biggest disadvantage to placing images on your web site is something completely beyond your control - images are only as good as the monitor that displays them. Resolution is controlled completely by the hardware and the visitor. While there are no absolute universal standards, selecting your palette from the web-safe color palette will give you the most control and enable you to anticipate how colors will be displayed on most browsers and in different environments.