Both the Gallery and the Slideshow contain pictures. They contain the same pictures. The difference is in the presentation.
Some people like to sit back and watch things change, while others like to hunt and click for what they want. The Slideshow is for the person who enjoys watching the pictures change on their own, and the Gallery is for the hunters in the crowd.
Both the Gallery and the Slideshow do have controls, however.
The Gallery allows you to filter on a particular job type. You can click any job type and the enable button, and only Gallery Items that have been tagged as that type of project will display. You can use your shift key to select a block of types, or the control key (on windows) and the command key (on mac) to select non-contiguous multiple selections. Just hold down one of those keys while you click the Project Types you would like to view pictures for. When you have selected them all, click the "enable" button - and enjoy!
The Slideshow has the same control area as the Gallery, plus links to pause the slideshow, and advance it in either direction. You can get through the photos much faster with the slideshow; but, if you want more information on the projects, you should poke around in the Gallery, as it links to content on each project, not just the pictures.
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