I just created the lolmogz community.
Being lazy, I grabbed the "Vector Drips" theme currently listed under "featured", altered the background header image, and changed the custom CSS to point to my altered image:
I've tried specifying the height and width, but exactly the same thing happens. My header image is 940x200, but it's being scaled down to 800x170, and then tiling to fill out the space.
I'm totally stymied as to why, since I used the theme's original header image and just altered some pixels!