Uploaded image gets resized

I'm using the Sunset Hair theme and replacing the "long hair" image with my own.  According to "Page Info" in Firefox the existing image has a resolution of 1,349 x 768.  If I upload my own image with the same resolution only a section of the image is used as the background.  Where am I going wrong?

EDIT 1

I've re-installed the theme, that seemed to sort out the "using only a section of the image" issue.  However, even if the uploaded image is sharp, it seems to get zoomed to fill the screen - in my case 1,920 x 1,080 - and then looks pixelated.  Is it possible to limit the maximum horizontal resolution of the theme?

Hi,

Depending on the setting for your background image, it can either be a fixed size or stretched to fit certain dimensions.  If you would like you image to remain the same size as it is when it is uploaded, in the form designer, set the background image to "Fixed":

This will ensure the image stays the same size.  If you set the image to "Cover", it will cover the entire size of the page.