Martin, you have done well by getting this far. I spent perhaps 1-2 weeks creating the custom front page, and it's harder for you since you have to understand the CSS codes that someone else wrote. Keep in mind, the complex custom header/graphics was tweaked specifically for the text/image we are using.
As far as I can see, you have the image intro plugin enabled, set to height:0, but it is adding some padding.
We do not use the image intro plugin on that page, although we may be using some of the CSS directly in the custom content. When I remove the empty image-intro plugin, it seems to work fine:
For your convenience, I have uploaded the entire
www.photo.gallery for you:
www.photo.gallery.25.sep.2018.zip [50MB]
It only contains the /content/ and /config/ directories, so you will need to have X3 installed. Simply replace the existing /content/ and /config/ dirs.