ed_f wrote:it's what I try and try _ captions won't go ... (I do not want that generally set, but for certain galleries)
From where are you trying that? Page settings > Gallery > Popup > Show caption:
I have now disabled captions:
https://www.architekturfoto.de/orte/berlin-fifty-fifty/
As you can see in the same settings, you can also disable specific caption options. For example, in your case, you are using TITLE (enabled by default), and the title will get popuplated from the file name unless a unique title is set. However, since you only have title and want it removed, you might as well just disable the caption.
ed_f wrote:one more thing: for this gallery with its images with white frame I want another image without the frame as preview - don't understand how to do that ...
Well, for X3 to automatically display a preview image from your uploaded image, it will have to create one from the uploaded image. You can't just "show different images, but point to the same original image". Furthermore you would have more problems: When clicking an image, how would the zoom-effect look? Pretty ugly, since it will zoom between two images that are different, and even have different aspect ratios.
This is not logical. There are ways to achieve this, but require a lot of manual work on your side for each image. It's not something any gallery website can do for you.