The problem:

You have something like this:

Fig 1. (click on image to enlarge).

You want something like this:

Fig 2. (click on image to enlarge)

Or maybe you want to modify the wallpaper on the left side of the screen slightly, but not the right. For instance, you might want to end up with a 'workspace' area to put your files in for extra tidyness, like so:

Fig 3. (click image to enlarge)

The solution:

I will demonstrate this on my desktop. This is a standard dualscreen (nvidia twinview) system; my main monitor on the left is currently at a resolution of 1680x1050, and my laptop screen on the right which I use as my second twinview monitor is at 1440x900.

Note: In the latest versions of gnome, if you have a dualscreen setup and you zoom or stretch a wallpaper, it does so for each monitor separately, which actually is quite useful. (It used to be that it would stretch it across both monitors which looks weird). If you're using an old version of gnome which exhibits the old behaviour, you can use the same GIMP principles we will demonstrate here to tweak your wallpapers to look exactly as you want them to. This process might seem a bit convoluted to you at first, but once you get the hang of it, it only takes about 30 seconds longer than your normal background changing procedure :)

The procedure consists of 3 steps:

1. Take 2 screenshots, 1 for each of your monitors
2. Combine in GIMP to create a single larger wallpaper
3. Set new image as wallpaper in 'span' mode

 

Ok, let's start.

 

Step 1

Suppose your current desktop looks like Fig 1. above, and you're trying to make it look like in Fig 2.

We need to 'empty out' the leftmost screen of anything that obstructs the wallpaper. Therefore:

• Set your wallpaper to be whatever you'd want to appear on the left monitor
• Move all your icons on the right monitor
• Right click on your panel, select 'Properties' and check the 'Autohide' tickbox.

Now, press your 'PrintScreen' button (make sure your mouse cursor is on the right monitor when you do so, otherwise it will show in the screenshot). Call this image 'left.png' or something like that.

Now, similarly we need to 'empty out' the rightmost screen, so:

• Set up your wallpaper to be whatever you'd want to appear on the right monitor
• Move all your icons back to the left monitor, leaving the right one empty.
• You can also undo the 'Autohide' business now at this point if you want.

Move your mouse cursor out of the way back into the left monitor, and take another screenshot; name this 'right.png'.

You should now have two screenshots that look something like this:

 

Fig 4. 'left.png' and 'right.png' respectively (click to enlarge)

Step 2.

Open one of those two screenshots, let's say 'left.png' using The GIMP.

For clarity, rename the layer as 'left.png' (Right click where it says 'Background', click on edit layer properties)

Find 'right.png' on your desktop and drag it into the layer toolbox. Your GIMP Layers dialog should now look something like this (highlighted in the image):

Fig 5. Layer's dialog

(Note that if you don't have a Layer's window, you can retrieve it by going to the main image window and clicking on 'Windows' → 'Dockable Dialogs' → 'Layers' )

Next, enable transparency by selecting in the main image window: 'Layer' → 'Transparency' → 'Add alpha channel' as in this image:

Fig 6. Add transparency

Now, make sure you have the 'right.png' selected in the layers, and that it is on top of the 'left.png' layer (if not, just drag it above the 'left.png' one in the Layers box. Then click on the 'Selection' tool from the toolbox. Make a small random selection anywhere on the image, then in the lower part of the tool box enter the dimensions of the selection to match your left monitor's resolution, i.e. in our case 1680x1050. (make sure to press enter or tab in the toolbox's textfield after you've entered the numbers to actually activate the selection.

Fig 7. Selection dialog

Then click on the main image window again to make it the active window (click on the titlebar, so that you don't accidentally mess up your selection) and press the 'delete' button. This converts your selection to a transparent one, essentially revealing the leftmost portion of the 'left.png' layer below, which is what you want.

 

Step 3.

That's it. Save this image with a name of your choice, and open it with 'image viewer', and set it to be your desktop background. When asked about 'style' select 'span', and there you have it. You now have a left monitor with one background, and a right one with another. Obviously if you change resolutions the illusion will break, but it's easy enough to remake the wallpaper for your new resolution :)

 

Edit: Just in case anyone wants to create a 'workspace' area like the one I did in Fig 3. I'm not going to go into too much detail, but these are the steps to follow in the GIMP:

• Make a separate transparent layer over the wallpaper screenshot you created and work in that.
• Create an appropriate selection over where you want your workspace to appear
• Use the 'rounded rectangle' option from the selection menu. You can 'feather' it a bit as well.
• Used the 'fill' tool to fill the selection with black colour.
• Adjust the opacity of the layer to whatever you want (it was set to 15% in Fig 3.).
• Save this as an .xcf file, so that you can fiddle with the 'workspace' if you want to enlarge it at a later stage etc. Then save another copy as a png file and use that to set it as a background (unfortunately the gimp default xcf file format with layer support cannot be used to set it as a background).