1920:1200 is 1.6, or 16:10 (reduces to 8:5, but represented as it is for the sake of people who know 16:9).
16:9 is 1.77777...., while 1366:768 is 1.7786[blah blah blah], so 1366x768 is roughly equivalent to 16:9. (1360:768 is actually 1.770[blah blah], so isn't quite as close to 16:9). The difference between 16:9 and 1366:768 should be nearly imperceptible, and might depend more on your display hardware than the original image itself. [Pixels on the screen itself may or may not be square, the graphics card output may be slightly stretched/compacted, the display may be set to stretch/compact the image, etc].
You could redownload the images, such that someone else cropped & resized for the 1366x768 resolution, or you could just use any other larger 16:9 resolution.
Or you could crop & resize them yourself.
Using one of the built-in Windows options for how the wallpaper is displayed will lop off the extra at the top and bottom of the picture, essentially soft-cropping it for you & resampling it to fit the lower resolution. Re-downloading the images could result in a better quality (maybe all the lower resolutions were resized from a large master image of a resolution higher than 1920x1200, or the resizing method was more high-quality), but probably won't be absolutely necessary.