Oh, that did the trick! I put the width in my css together with height=auto.
In my WordPress page-code, I added width and height of the picture and voila: Works like a charm! Thank you!
My code before (which didn’t work):
[gallery reel="1" size="full" ids="573" loops="false"]
My code looks now like this:
[gallery reel="1" size="full" ids="573" width="980" height="350" loops="false"]
