Hi MathiasB,

The lightbox image JavaScript used in concrete5 is Magnific Popup. Magnific Popup will read the title attribute value of an image and then create an image caption from it.

Adding a caption using the File Manager file description would probably involve overriding the core redactor.js file. The override would change what information ConcreteFileManager retrieved and a change in the modal template for a title field or the information retrieved was applied automatically as a title attribute value.
concrete\js\build\vendor\redactor\redactor.js

There are other options for this, but all involve code.

An alternative is upgrading to version 8.2.1 of concrete5, it doesn't automatically get the file description, but does offer a title field for adding caption text for lightbox images. If you do upgrade, be sure to make a full backup first (database and files).

Dear MrKDilkington
You again... :-) Thank you so much for this information.
I guess it is too difficult (and too risky) for me to edit the code of the redactor.js. But at least, I know where it is now.

Do you know somebody professional (maybe a core-team member) who could help me to solve that?

Best Mathias

@MathiasB

Are you looking to hire someone to add this feature for you?

exactly, but of course it depends on the costs... I you could edit the js-file for me, please send me a privat massage with offer (paypal). I think this is not the right place for that kind of negotiations ;-)