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.