Random inline graphics are a great way to add appeal to any Web document; however, they're not the only thing you'll need to do to keep your Web pages fresh. Studies show that visitors to dynamic Web sites expect things to change on a 30-day (monthly) cycle (think magazine subscription).
It's not just the graphics that need to change. Keep your dynamic sites alive by frequently adding new articles of interest and creating additional pages with information relevant to the site. You get the idea.
One more thought: Your index, or home page, is the gateway to your site. Like the cover of a monthly magazine, your site's entrance should change on a regular basis and give visitors links to new information.
That's it for now. Keep learning, and take it to the Next Level.