If the image for a particular asset does not have the correct thumbnail it can be re-generated by deleting the existing thumbnail images. 

Thumbnails generated for Media for use in the preview pane of ~Sedna Presenter are placed inside the Thumbnails folder within a .presenter file.

How to Re-Generate Thumbnail Images

1. Close the project file which is experiencing this issue

2. Browse to the project file in Finder

3. Control + Click and select "Show Package Contents"

4. Double-Click the Resources folder

5. Double-Click the Thumbnails folder

6. Delete the Thumbnails Images within this folder

7. Close the Finder window and re-open the project in ~Sedna Presenter

The thumbnails will now be re-generated when clicking on the Asset within presenter.