People have been trying for years to get a movie of Arthur C. Clarke's Childhood's End off the ground — and there has been some amazing artwork depicting Clarke's Overlords. Including a design by creature-designer extraordinaire Wayne Barlowe, and drawings by Batman artist Neal Adams!
While working on the post about "entry level" science fiction books, we stumbled on a treasure trove of Childhood's End art over at Infinispace. Check out some more concept art which Neal Adams created for an abortive film version below, and there's tons more at Infinispace and Neal Adams' website.
And here's Wayne Barlowe's artwork of an Overlord, which comes from his classic book Barlowe's Guide to Extraterrestrials:
And here's the Overlord Karellian, by artist Paul Harmon:
And finally, Naenae Suwankiri's art has a distinct Guillermo del Toro vibe: