Does this attribute simply enhance semantics?
Yes, it’s about semantics of your content. That way, your page/content is part of the Semantic Web.
foaf:Image (most likely) refers to the Friend of a Friend vocabulary/ontology. The namespace
foaf: typically stands for the URI of that vocabulary (
http://xmlns.com/foaf/0.1/), so the value is actually the URI
http://xmlns.com/foaf/0.1/Image is a class. It’s (= what it means) documented here: http://xmlns.com/foaf/spec/#term_Image.