Settings

You can open Options from Add-ons/Extensions page in FireFox/Chrome or by pressing Ctr+O in slide show or viewer mode.

Slide show options

Auto start slide show otherwise you need to press Play button to start it.
Repeat mode. If it's not checked slide show will be stopped on last image.
Show control panel ONLY when mouse over it. Otherwise it will be displayed when slide show is over or you press SPACE (pause shortcut).
Slide show delay in seconds. Real timeout can be greater because of internet speed.
Minimum width/height of image in pixels. if image width or height is less than this value and there is no link to big image and there is no special Rules then it will be skipped.

Auto image size

Shrink images to fit screen otherwise it will be centered.
Zoom images to fit screen otherwise it will be centered.

Viewer options

Scroll step value is used to scroll big images.
Zoom by value is used to zoom in/out images. It's always percent of original image size.

Miscellaneous

View image in full screen when you click on link to image. If you click to link which point to image then image will be open in fullscreen mode instead of new tab or new window.
View image in full screen when you click on thumbnail according to rules(FireFox and Chrome). If you click to thumbnail or link which have special rule then image will be open in fullscreen mode.

Description of Rules tab see in How To Create Rules section.