Should 've read the last part of the description on the WP-plugin page better - the 'Chinese' I was referring to earlier is clear code here... That gave me one part of the solution. So, in case anyone has the same question, this is the solution:
1. Put the main image in your post in the usual way (don't forget to link it to the mediafile)
2. add rel="lightbox[groupID]" to the first part of the a tag ( 'groupID' can be any name you choose for the group of images you want to link).
3. Add the other images to your post the usual way (again: don't forget to link it to the media file)
4. Delete the actual <img ..... /> part inside every link of these other images, so you end up with only the a-links (that won't be visible in your post).
5. Add the same rel="lightbox[groupID]" to every one of these links.
Et voilá... When you click the image in your post now, the lightbox opens and you can click through the whole group of images.