The iTunes API doesn't provide a way to grab artist images, but the iTunes website uses Open Graph meta tags, which embeds a meta tag with a property attribute value set to og:image. As it turns out, this seems to be the same image used in the iTunes artwork.
The URL structure is similar to the artworkUrl values returned by the API, but what concerns us here is the part I've indicated at the end of the URL.
http://is3.mzstatic.com/image/thumb/Music7/v4/68/68/41/68684190-833b-bfb4-5018-e5a2e6f69eb0/source/1200x630bf.jpg
└─ widthxheight