Changing Navigation Background Image Size

I'm using GeneratePress with the GenerateBackgrounds-AddOn.
I'd like to use the same background image for the footer and the Navigation bar.
In the footer area, it is possible to resize the image so that it is not stretched to fit but repeated. I'd like to have the same way in the navigation bar, but there is no possibility to resize the background image of the navigation bar. Instead, the image is stretched and distorted in the navigation bar.
Is there any way to change this?

Thank you for your help!
Therese