I like having fully-spanning wallpaper on my three screens. 3840x1024 images are what you need, but getting a detailed photo at that resolution is hard (you really need a ~20MPx camera with a very expensive lens). Here are some that I've made, mostly using a 10MPx camera, occasionally with some stitching.
I make no claims as to any artistic or technical merit in any of these.
Neuschwanstein Castle. Composite of about 10 10MPx photos (with a wobbly horizon) so the detail is very good.
You know what this is. Composite of 3 10MPx photos.
北京国家体育场. A single photo, at night, but the detail is not bad.
A sign in 河南博物院.
These images are free for personal use. Contact me for permission first if you want to put them on some other site. Please don't link directly.