Hi Andy,
thank you so much for your quick response.
That plugin worked just perfectly. :)

First, it didn’t show my text. But then I changed sth in the following file wp-content/plugins/lightbox-plus/classes/filters.class.php so that the alt-text, instead of the title is shown on my page.
Maybe someone has the same issue. I found it under the following line:
on /** Do Not Display Title is select – yes/no **/,

After that there was a second problem. Instead of only one Lightbox, it showed me 2 Lightboxes. I fixed that issue by just deleting the “z-index” in the following file wp-content -> plugins -> lightbox-plus -> css -> black -> colorbox.css in line
#colorbox, #cboxOverlay, #cboxWrapper{position:absolute; top:0; left:0; z-index:9999; overflow:hidden;}

Now it looks exactly how I imagined it. :)
Thank you once again.
Have a nice day.