Erin:
Archiving an asset will not delete it or change it's URL location. Pages that have these assets (e.g., image) will keep the images on them.
Archiving assets only pertains to the Marketo organizational structure. The file still exists at the same URL, archived or not.
Archiving an asset means that it will not be available in the drag-and-drop asset menu. As soon as you archive the image, it will no longer appear as an option to "place" on a landing page or email.
If a page already has the image placed beforehand, then the image becomes archived, the editor for that page still shows the image. It can be moved, resized, etc., within that page that it is already placed.
As for a way to tell if an asset is currently being used on which pages, you CAN do this. If you navigate to the asset and click on it (the part that shows the full URL of the asset), it tells you which pages use the asset. This functionality remains whether the asset is archived or not.
Best,
Edward Unthank
SEO/Web Specialist