preview image changed from presentation to presenter 

**Version**
v2.16

**Describe the bug**
Preview image change from presentation to presenter

https://github.com/elan-ev/studip-opencast-plugin/commit/7e77261de44fb9ba4f4429d7d4c8ab0c4bedf566#diff-3501aa8d29973fc977b3b93a568d0381e2c84eff567b3ccd82714b9cd747189cR546

Was the change intended? 

