I don't understand. Your code is just:

There is no iframe to be found.

If you want an iframe using this, you just do:

<iframe src="http://www.ultrahdwallpapers.net/file/1876/3840x1024/crop/cat_staring-wallpaper-3840x2160.jpg" width="3840" height="1024"></iframe>

You will not be able to have it so big without scrolling unless you crop it or scale it down.

Why now download it, resize it and then upload to your site?