Already created text collections cannot be converted to image collections as the text links do not contain any image information
If a text collection has been created, if converted to thumbnails, it will only display the "no thumbnail" frog image and nothing else.
You can always create thumbnail collections as you see fit. For a temporary solution to your situation you can just add another thumbnail collection under the already inserted text collection so readers can add their link again, this time using their images as well..
You can let them know if you want ,by going to your dashboard and clicking "emails only" on the old collection. This will give you their emails so you can send them a notification that they can re-enter their links with a thumbnail now.