by Marina Glancy.
Ok, then I probably misunderstood you. All repositories included in moodle standard package already display thumbnails. I thought you were talking about the repository you developed yourself. If not, you need to talk to the developer. You said "I created a repository instance" but you did not specify instance of WHICH repository type.
If you are not the developer there is no way to add thumbnails without modifying the code, sorry.