Image Creation for 3 monitor setup

[seaweavle10]

I have 3 monitors, each 3440x1440 resolution.  I've never used any image editing software before, but I have 3 images that I would like to have as my desktop background (one in each monitor).  I've watched a bunch of tutorial videos on gimp, but can't figure out how to get all the images into one canvas where they would be centered on each monitor.  Also, the images are all different sizes, which is making this difficult.  Not really sure what to do. Any suggestions?  Thanks!

[Ofnuts]

If you three monitors are seen as one huge monitor, then just create a 10320x1440 image, and add 3 images that you each scale to 3440x1440. But in the multi-monitor systems I have encountered so far, you can define a background image for each monitor, so you don't need to group your images into a big one.