Text over header image

Hello everybody,

I have a Twenty Fourteen homepage (version 1.2).
The link is: http://toekomstboeren.nl/
As you can see we have a header image with some text on it.
At the moment it is all an image but I would like to be able to change the text easily in future, so to have the image and text seperated from each other which makes it easier to maintain.
Is this possible and if yes, how?
I am already working in a child theme and also did quite some css changes, still please be clear about where I should put the css code (assuming that it’s possible!) header.php oder whatever..

Thank you very much in advance!
Looking forward to hearing from you.
Cheers
carlchen