Starting with a blank canvas, you can drag your mouse to the bottom right corner and click a gradient color for the background, or upload an image to create your own background.
For my example, I chose a picture file that I created which I thought would be a cute background. I clicked on the top left corner on the 'add picture' button.
Double-click and there's the background!
You can add all your pictures at one time by clicking that top left "add picture" button.
It's easy to move the photos around. Just click and drag. Then click on the individual photo text to give the photo a name or simply delete the text.
Export as wallpaper or an image (or whatever filetype you want) by clicking on the top right corner "Export" button.
Fotowall is available for free through the Ubuntu Software Center.