1. Import your image as a tiddler (see Images in WikiText)
   • You can also use an external image
2. Open the control panel and switch to the Appearance/Theme Tweaks tab
3. Select your image from the dropdown labelled "Page background image"
4. Set "Page background image attachment" to "Fixed to window" to have the background stay stationary and the content to scroll over the top of it, or "Scroll with tiddlers" to have it move (note that the iPhone/iPad doesn't support the fixed setting for performance reasons)
5. Set "Page background image size" as follows:
   • Auto causes the background image to be tiled over the page background
   • Cover causes the background image to be sized so that it completely covers the page. Some of the image may be clipped
   • Contain causes the background image to be sized so that it fits within the page

Note that the palette DarkPhotos is provided to make the sidebar more readable on dark background images.