For iOS and Android enabled publications, users can set a thumbnail for the video on the page before selecting to play the video.
Note - the thumbnails must be uploaded to the publication directly, not the group folder
To select a thumbnail, first of all navigate to the Files section and upload a .PNG or .JPEG image of any size or dimensions using the upload facility.
Next, navigate to the Settings tab, then Thumbnails.
The uploaded file(s) will appear in the drop down menu next to the relevant video(s) you have uploaded - simply select the image from the drop down menu and hit Submit to save the changes.
Remember to Regenerate your preview link to see the changes.