Here are some examples demonstrating different kinds of galleries work.

Swap Image Gallery
Click these thumbnails to swap out the image on the right for a new one. When the mouse is moved away from the thumbnail the gallery returns to normal.

New Window Gallery
Click the thumbnails to open the images in a new window. Text can be added to the new window to describe the image.

Scrolling Gallery
Use the toggle to th right of the image below to scroll through a series of images. Text can be added within the scrolling area to describe the images.

Red Rhino Yellow Rhino Blue Rhino Green Rhino

Scripted Gallery
There are many flash and javascript galleries that can be downloaded for free. Many can be customised to suit your website perfectly.

Image Protection
The swap image technique can also be used to protect images. When moused over the image is replaced with one that is marked.