This is an easy one, but might take some people sometime to figure out.
Problem: The admin run into an problem today when switching an application pool for a site. The site was not rendered correctly once switched to the new application pool and with the old one stopped. Upon taking a look, all the images were not showing.
Solution: Make sure the 'images', along with all other applications (virtural directories) are using the same app pool. In this case, the new app pool.