This whole ordeal is facilitated by the .desktop -files that you can find on your N900. You put the filenames of four separate image files (I've used only JPG:s, but I guess PNG:s would work too) into one wallpaper control file (with the file extension .desktop).
The easiest (?) way to put a wallpaper on your N900 would then be the following:
Copy a .desktop file from your N900 to your computer (by connecting the N900 with a USB cable or by SFTP:ing in to it, if you have openssh installed). You can find the .desktop file in the .images folder. Just choose any of them, let's say .abstract1.desktop.
Note that with a Mac computer, you won't be able to see the .desktop files in finder, unless you tell your mac to show hidden files, which is not that easy. You're almost better off going the SFTP way and using Fugu or such to connect to your N900.
Get the photos (you could take the four photos from my link or any other photos from n900wallpapers.com). You could also make your own 4-part-wallpaper by starting out with one 3200x480 panorama image and cutting it down to 4 separate photos that are exactly 800x480 pixels or you could just pick out four different images at exactly 800x480. Your choice.
Edit the .desktop file to reflect your images. Let's say you downloaded my forest girl -photos and renamed them forest1.jpg, forest2.jpg, forest3.jpg and forest4.jpg. Start by renaming the .desktop file so it reflects the contents of the wallpaper, for instance .forest.desktop.
Then open up the .forest.desktop -file in your favorite text only editor (Notepad, for instance). Mine looks like this:
You should change five things: the Name -row to reflect the name you want for the wallpaper and the four rows containing the reference to the JPG-picture.
If you left Wallpapers/ in the file name path of the four images, you should create a folder called "Wallpapers" under .images and upload forest1.jpg - forest4.jpg to that folder. Otherwise just upload the images to the root of .images. The most important thing is, of course, that the paths in the .forest.desktop -file point to where the images actually are.