Featured image for teasers?
I would say the easiest way is to make the first image also be the "featured" one, as then you already have most of the functionality built in.

It is then trivial to use a view that filters on "Image field: delta = 1", to give you only that one image, and then output it where appropriate.

A php savvy themer could also pull out the first image right in the template as well.