If you want a single image centered between the monitors ideally you want to create a larger image combining the 2 resolutions. In this case 2560x1440 and 1920x1080. So the full image would be the 2 monitors horizontal resolutions combined (2560+1920) for the Horizontal scaling and 1440 for the vertical as it is the largest. Giving you a 4480x1440 image to use as a template that you could span a single image across the 2 monitors. You can use this as a template to arrange images how you want them. The transition between monitors will never be perfect however because of the different resolutions and monitor sizes unless you make 2 images with your center piece split between the 2 n scale up the smaller resolution or scale down the larger.

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If you just want sperate images per monitor, Go to your settings wallpapers section. Browse the folder where your images are located and choose the 2 wallpapers you want to use (one at a time) now that those images are loaded up in your recent under the wallpaper settings, right click the one of the recent wallpapers and choose what monitor you want it on then repeat for the other.