The thumbnail for the images cannot be changes easily. It is set from the LibraryHttpHandler and the Thumbnail property of the MediaContent does not have setter. You can change the thumbnail of the Videos only.
You can control the spacing by modifying .sfimagesTmbList .sfimagesTmb css classes with an important statement. You can declare these css classes in your theme stylesheet and they will override the base classes.
To create your own custom control you can use the fluent API to get the items
Then you need to create your own custom widget
and implement the selector and UI for it.
the Telerik team