Sites like HDWallpapers and Mob.org provide categorized galleries where you can select the 1366x768 resolution directly before downloading.
For dual-character art, AlphaCoders offers high-resolution downloads that can be cropped or scaled to 1366x768. Download 1366x768 Arknights, Hoshiguma, Horn, G...
Here are the best sources for these specific characters and resolutions: Sites like HDWallpapers and Mob