Why should you download a web browser and a screensaver separately, when you can both of them together? This is the main idea behind FoxSaver, an extension for Firefox that builds a screensaver inside your browser.
Whenever your computer is idle for a period of time (which can be configured in Settings) FoxSaver launches a photo slideshow that works as screensaver. The extension can take pictures from several sources, including the browser's cache, the photos stored in the hard drive and also pictures uploaded by the extension's developers, although you can't preview them in any way.
FoxSaver features some configuration options that enable you to customize the screensaver, although I missed having some transition effects among them.
With FoxSaver you can show off your own pictures in your favorite browser.