On windows XP it's possible to make desktop to display html content and thereafter add a several images on it, you can move and resize them. So the basic idea is that you place the images you like on your desktop, place them where you like, resize them as you like, then lock them, so they became part of the desktop image.
How this can be done in ubuntu (xubuntu) ?
There are 2 ways to achieve this, firstly you can create a slideshow which will transition multiple images 1 at a time. See How do I create a desktop wallpaper slideshow? on how to do it.
However, if my understanding is correct, you are asking how to create a customised background with multiple images.
To do this you need to use an image editing software (like GIMP) with layering capability. In this software you can build your custom background as you like with multiple image with the ability to place, resize, rotate etc. You can the "lock" the layers and use the image you create as your desktop background. FYI, GIMP is available from the ubuntu software centre and is quite difficult to use for a novice.
