Safari stores PNGs and JPEGs of web site previews in your Home directory "Library" folder (Exact path: ~/Library/Caches/com.apple.Safari/Webpage Previews). These image files are used by Safari in the Top Sites and History section of the Browser.
The images are usually a thumbnail of the entire page previously viewed by a user saved as a JPEG and a full size image of the Web Page saved as a PNG. So for example, if you have 1914 files in this folder, you actually have 957 same previews of web pages (two images per page) viewed by the user.
Safari is constantly creating new previews in this folder and removing older files as needed. Each file will also have a creation and modification date that will usually tell when the page was viewed. When looking at unallocated space, you may also find many images that have been deleted over time which may show important historical web activity. It is a great place to look when trying to understand what a user might have been doing on a Mac or even a Windows system using Safari.