I was surfing the net on Google to search for wallpaper sized Thomas pics or larger when I came across this high-resolution image from a Lionsgate website:
As you can see, it is very large! The image has a background with a superimposed large scale Thomas which looks as though it is meant to be from the 2008 episode, The Man in the Hills.
The image is exactly 1680 x 1600 in dimensions size and is a JPEG. if you want to use it as your desktop wallpaper, you can easily reduce the size and crop it.
If used in videos on YouTube or any other sites, please give credit.
Have fun with the image!