Hi Manuel,
thank you for not only reporting this, but your effort in trying to find the source of the problem.
I had a look at https://www.manuelfink.de/blog/2014/test-image-src-control/
* the image is growing out of the post, because it is right floated. The ISC container inherits the align-parameter from the caption and you set it to “alignright”. Remove this and it sould work.
> It seems that title images of themes (e.g. banners over the text) are not supported at all or?
> Nor I think images within page builder are supported?
> Also it’s not showing sources within the jetpack mosaic galerie.
Right, the source caption is only possible for images with posts. There are just too many exceptions for other behaviour out there, but I welcome everyone who can support the one or another solution. The source code is publically visible on github too.
> Do I need to resave old posts that the sources are added?
No need to do that.
Regarding the screenshots. I don’t really get the problem. If you like, you might explain in German ??