Hi Doug,
thanks for your reply. I used the CSS code as suggested in this forum:
#intro #intro-background { background: url(%resource(name-of-your-image.jpg)%) no-repeat center center; background-size: cover; }
I have just tried replacing the actual header photo in the theme data (as suggested here: http://forums.realmacsoftware.com/t/change-the-banner-background-in-tesla-split-theme/3343/85), which works as well.
I did try various ratios as per your suggestion, but the effect is the same. I have these bars appearing above and/or below my image. Thanks for the hint about file size though!!
Alex