OG Images lists the templates by kind of page: home page, items, collections, files, exhibits, exhibit pages, simple pages, blog posts (with the Blog plugin) and search results.
The default template
Each kind of page uses its default template, marked Default. Make default next to another template switches to it. Installing OG Images creates one default template per kind; when a kind has none (all deleted), a built-in layout is used.
Adding and duplicating
Add a template under a kind of page starts a new template from the built-in layout. Duplicate copies a template, to try changes without touching the one in use; make the copy the default when it is ready.
A template can be moved to another kind of page by changing For in the editor. Variables of the old kind then give nothing, so check the preview.
Deleting
Delete removes a template. If it was the default, another template of the same kind becomes the default.
Seeing an image
See an image under each kind opens the image of the newest page of that kind, as networks get it.