Add JPG and PNG media types

[rjyounes]

Two commonly used image media types:

@prefix media-image: <https://www.iana.org/assignments/media-types/image/> .

media-image:jpg
	a gist:MediaType ;
	skos:prefLabel "JPG"@en ;
	gist:uniqueText "image/jpg"@en ;
	.

media-image:jpg
	a gist:MediaType ;
	skos:prefLabel "PNG"@en ;
	gist:uniqueText "image/png"@en ;
	.