The Default Wallpapers
The trinity-base and trinity-artwork packages include a variety of wallpapers in different sizes and resolutions. Most recently, several landscape pictures appropriate for modern widescreen monitors were added to the selection. An additional wallpaper selection is available through the "Get New Wallpapers" button in the desktop backgrounds section of the Control Center.
In addition to normal static wallpapers, Trinity can use an image dynamically drawn at intervals by a program as a background image, and by default offers a clock and a Web-based desktop background. The options for this can be found under "Advanced Option" in the desktop backgrounds section of the Control Center.
Using Non-Trinity Wallpapers with Trinity
Trinity can display wallpapers in a wide variety of formats, including the commonplace JPEG, PNG, GIF, and Windows bitmap (.bmp). It also supports a variety of more exotic file types (TIFF, PCX, XBM, etc.) Basically, almost any wallpaper from any site on the Internet should work, although it may be necessary to adjust the options for scaling, tiling, and positioning to get exactly what you want.
A selection of wallpapers and wallpaper sites recommended by Trinity users can be found in the Community section of this Wiki.