use xarchiver to unrar (or just unrar from a PC and copy them into your n900).
after extraction, you should see a .desktop file and 4 images. there is one more image called preview. you can ignore the preview image.
copy all the 4 images and the .desktop file into /home/user/Images.
then go to your desktop, tap the top to edit the desktop, in the menu, select "change background" and you should see the image set appear in the list of backgrounds.
I managed to change the background to this, however, in portrait mode, I still get a black stripe between the desktop views, the same as with other wallpapers! Have I done something wrong here?
Set the wallpaper while your phone on the landscape mode, then u may flip it portrait once you're done setting it. (because setting it on the portrait mode may give bad images sometimes)
which wallpaper is that? or are u having trouble with them all?
and plz paste for me the content of the .desktop file u used. (on phone: using file manager, by clicking edit on the file, then copy the content and paste it here)