header images size and custom header

I am working my way through the themes to change the header image in each.

I am monitoring and contributing to another thread attempting to find a fix to the custom-header.php failure on my site. The thread is at https://premium.wpmudev.org/forums/topic/customisation-doesnt-work-on-magazine-social-nelo-triden-dixi-premium-themes

In order to create a consistent look, I am manually replacing the default image in each of the themes I am offering to my subscribers. As I do this, I have found that the first few I have worked with do not return the correct image size needed to work with the theme, or the theme is not resizing properly.

For example, in the WPMU-Triden theme, the custom header page shows that the image that is uploaded must be 960 x 150 pixels. However, when I uploaded an image of that size to my site, the right side was cropped. I was only able to get the full width of the image to display by reducing the width. In this instance, I wound up using an image of 880 x 130 pixels.

I am not a css expert, but in looking at the css, I think I saw where the container or body was set at a percentage of the screen.

Wouldn't it be more correct if the css also resized the image to fit the container or body?

Will this be a problem with all the themes?

Or, am I misreading?

And, is it snowing and I will be trapped in the house for the next three days?