UKU – Custom Fields

Hello Elmastudio,

I have purchased Single Theme UKU. I’m using clear WordPress 4.7.3 (one post, one static page – default thema, one top menu and one plugin Regenerate Thumbnails) to test new Uku thema.
Unfortunately I have huge problem to show a big Featured posts image by adding a Custom Field.
I have followed your documentation instruction, loaded one single image greater then your recommendation (1440 x 690), used Regenerate Thumbnails and loaded as a featured image in my single published post.
I can’t achieve the same effect which is shown in the seventh paragraph of the manual (http://www.elmastudio.de/en/themes/docs/uku/). My post just not show up at all.

Would you please advice me what can be the issue here or provide me with step by step instruction?
I would like to also know more about capabilities/advanced possibilities of custom fields?
e.g. How many posts can be published in this way, can I chose posts to published?

Best Regards
Greg