The foaf:img property relates a foaf:Person to a foaf:Image that represents them. Unlike its super-property foaf:depiction, we only use foaf:img when an image is particularly representative of some person. The analogy is with the image(s) that might appear on someone's homepage, rather than happen to appear somewhere in their photo album.

Unlike the more general foaf:depiction property (and its inverse, foaf:depicts), the foaf:img property is only used with representations of people (ie. instances of foaf:Person). So you can't use it to find pictures of cats, dogs etc. The basic idea is to have a term whose use is more restricted than foaf:depiction so we can have a useful way of picking out a reasonable image to represent someone. FOAF defines foaf:img as a sub-property of foaf:depiction, which means that the latter relationship is implied whenever two things are related by the former.

Note that foaf:img does not have any restrictions on the dimensions, colour depth, format etc of the foaf:Image it references.

Terminology: note that foaf:img is a property (ie. relationship), and that code:Image is a similarly named class (ie. category, a type of thing). It might have been more helpful to call foaf:img 'mugshot' or similar; instead it is named by analogy to the HTML IMG element.