The first reason to cut down on graphics is that the more there are, and the larger they are, the longer it will take each of your pages to download. People are impatient when waiting for pages to download -- you only have around 5 seconds before your visitor hits the Back button.
What can you do about this? Apart from using fewer pictures, you can also make sure that you resize your images in a graphics editor. This actually makes their file sizes smaller. If you just resize images by specifying a width and height in HTML or CSS, they will still be slow to download because the full file size is being used.
You should consider turning on compression in your image editor. JPEG files can often be compressed by up to 25% before there's a noticeable difference in quality. Try different formats and compression levels to see what works.
It Gets Too Busy
If you use a site with more than 4 images on the page at once, your eyes are being pulled all over the page. They're not sure where to focus because the page simply has too much going on.
Look at the front pages of newspapers, and notice how they lead on 1 picture. Putting 2 pictures on a front page is considered to be poor: the reader doesn't know where to look.
That goes double for websites, where the viewable area is much smaller than a newspaper page. Even if you have more than 1 thing to say, it's better to 'go large' with 1 picture and then explain the other things in text, next to or below it.
It Distracts from the Content
Users visit your site to get information, not to look at your graphics. Too many graphics will distract from your content, or, worse, force readers to search for it. Any time your graphics get in the way of people readily using your site, you're suffering from graphical overload. And that is a bad thing.
What's the solution? Simply decide which of all those graphics are really necessary. Remember, don't add graphics just to look nice, each graphic must have a specific purpose.
An Exception: Photo Galleries
If the purpose of your site is photo presentation, then clearly multiple images are appropriate. However, don't just stick up several large photographs -- provide thumbnails: smaller versions of each image. If interested, the visitor can click on 1 to make it larger.
This fits more pictures on each page, and avoids wasting user download time and your bandwidth.
Keep in mind that in all web design, the images are there strictly to support the content. Even when the content is graphical.
