Various artists
Olga Tchubarova (Tenevaya Olga)
Artist's Flickr photostream https://www.flickr.com/photos/126735555@N08/.

Penrose triangle

Blue Jay

Loki

Humming-bird

Bird with Thread

Dragom-fly
Artist's Flickr photostream https://www.flickr.com/photos/126735555@N08/.