PSA: All of these links are NSFW.
Okay, I had three types of images I wanted to use as wallpapers. The most common, and most irritating, was where either length or width was larger than my desktop (1920x1080), such as the case of this image: https://ottrbutt.com/tmp/284c113f7df2443583a1d8a254ab96b0.jpg -- which is 1600x1200. Since 1200 is greater than 1080, the image would get cut off. To fix this, I resized it to the largest size that would fit, making it 1080 pixels wide. When reducing the width, you also have to reduce the height by the same amount, or the image looks fucked. 1200 - 1080 = 120, and 1600 - 120 = 1480, therefore, the size it needs to be is 1480x1080. I did it with Imagemagick.
The second type was images that had dimensions that were less than 1080p (https://ottrbutt.com/tmp/5e8c97e441b497dc89a6fdfe6abe170e.jpg), meaning they wouldn't get cut off, but because they were less than 1080p, tile would not work correctly. You would end up with part of an image on one page and part of it on another, making it look awful. This also usually applies to images that have been shrunk to avoid getting cut off. What needs to be done is basically padding it with black pixels until it's 1920x1080. The following command resizes the image to 1920x1080, centering it inside that area, and then making everywhere that isn't part of the image black.
The third type of image is one that is already 1920x1080 (https://ottrbutt.com/tmp/379892403fc13daf40141e7371a95b89.jpg), and therefore nothing has to be done to it. At this point, the image is ready to be made into a dual-monitor wallpaper with another image. The one on the left comes first, the last one is the output file.
These are the original images: https://ottrbutt.com/tmp/284c113f7df2443583a1d8a254ab96b0.jpg & https://ottrbutt.com/tmp/c114eda63b8d67b2d822740a977b3f9e.jpg
These are those images once shrunk to fit a 1080p screen (while maintaining aspect ratio): https://ottrbutt.com/tmp/284c113f7df2443583a1d8a254ab96b0-shrunk.jpg & https://ottrbutt.com/tmp/c114eda63b8d67b2d822740a977b3f9e-shrunk.jpg
These are those images shrunk with the correct padding: https://ottrbutt.com/tmp/284c113f7df2443583a1d8a254ab96b0-shrunk-padded.jpg & https://ottrbutt.com/tmp/c114eda63b8d67b2d822740a977b3f9e-shrunk-padded.jpg
And here are some final Windows wallpapers!
https://ottrbutt.com/tmp/win7-dualhead-wallpaper-1.jpg
https://ottrbutt.com/tmp/win7-dualhead-wallpaper-2.jpg
https://ottrbutt.com/tmp/win7-dualhead-wallpaper-3.jpg
https://ottrbutt.com/tmp/win7-dualhead-wallpaper-4.jpg
https://ottrbutt.com/tmp/win7-dualhead-wallpaper-5.jpg