I've got two 24" monitors that I use side-by-side in TwinView mode in my home office, so when I look for desktop background images I hit up Mandolux. Mandolux offers free and original wallpaper in a variety of sizes for even the largest displays.
The only wrinkle in all of this is that Mandolux splits the larger widescreen backgrounds into separate left- and right-hand images. However, since I run my display in TwinView mode, my desktop is essentially a single 3840 x 1200 display ( two 1920's side by side). For the Mandolux wallpapers to display correctly on my background I need to merge the two images into one. I could manually stitch the two images together with an image editing program like GIMP, but since I downloaded a dozen of the background images to try out, doing it manually sounded dauntingly tedious.
That's when I remembered that the ImageMagick suite of tools includes the 'montage' tool. The ImageMagick tools can be installed with the following command under Ubuntu:
sudo apt-get install imagemagick
Then, to stitch the images together I ran the following command:
montage mandolux-ga2k6-*-1920.jpg -tile x1 -geometry +0+0 mandolux-ga2k6-3840.jpg
in a directory containing mandolux-ga2k6-l-1920.jpg and mandolux-ga2k6-r-1920.jpg (left and right side images, respectively). It is also worth noting here that the first filename precedes the second filename alphabetically, so the files are laid out from left to right in that order.
The result is mandolux-ga2k6-3840.jpg, which is a file with both images merged into into one 3840 x 1200 image.