Picture should completely overlay a background on all screen sizes and mobile

Hey there,
spent my night to figure this out, but now I have to give up. It just doesn’t work and I need sleep. What I’m trying to achive is to have a solid picture with a big transparent letter in it (.png) so that the background picture is showing trough it. That picture should always overlap the background picture. Background should be fixed or parallaxing. It’s working on some screen sizes, but as soon as i decrease the browser window the background always becomes visible on the sides.
I just can’t figure it our and I’m crying for help. You’ll find login credentials in the private part. And as soon as you have a look at the page “Start” you’ll know what I mean and what problem I need a solution to.
Hope you can help me!
Thanx in advance,
tribaleye