My question, is it possible to have more than one actor tagged in the series field.
No.
As you will find if you look in the "Options > Library & Folders > Manage Library Fields" dialogue, the [Series] field is a String field, and so can only hold one value for each file.
The [Actors] field is a List type field, and so can hold multiple values. But there is no Cover Art linked to the Actors field. You could request that of JRiver, but actually, your Collections are not just based on actors, as shown by your Hitchcock Collection. So the [Actors] field isn't what you want to use either.
The [Artist] field is a List type field that can hold multiple values, has Cover Art associated with it, and is generally empty for movies. So it would be a perfect field to use for your requirement.
I'm not going to go through all the details now, but you could add to the [Artist] field for the movies in Collections the Actor, Director, or whatever that you use to associate items into a Collection. One item could belong to multiple Collections. You could then put an image into the Cover Art location (C:\Users\[UserID]\AppData\Roaming\J River\Media Center 25\Cover Art\Artists) named as per your Collection grouping item, i.e. Hitchcock.jpg, "Jimmy Stewart.jpg", etc.
Then create a View, limit it to movies, and make the first Category in the View "Artist". The View will then group all movies by each Artist value, and show the image you saved as Cover Art for the Collection.
Perhaps use the full Collection name in the Artist field and image name. i.e.
[Artist] field includes "Jimmy Stewart Collection" ==> "Jimmy Stewart Collection.jpg"
[Artist] field includes "Hitchcock Collection" ==> "Hitchcock Collection.jpg"
The Artist View you create would then show "Jimmy Stewart Collection" and "Hitchcock Collection" under the thumbnail image.
See how you go with that. Ask for help if you can't find info in the Wiki or Forum, and get stuck.
