(5 years and 1498 days ago)
Use one of the PxlEyes sources and Photoshop.
Thank you, Snappler for the awesome wooden texture: http://www.pxleyes.com/picture/10646/4aecc032e5d4d.html (5 years and 2209 days ago)
I couldn't resist.
The only source used in this entry was the original contest source image itself. I had included (mistakenly) a link to a Web page with a library of optical illusions for fun, but it was considered an illegal source (?), so I deleted the link, sorry.
The background was made with a greatly enlarged version of the source, with Polar Coordinates (Polar to Rectangular) applied twice, the Gaussian blur and blue color overlay.
One mosaic "cube" was masked and duplicated to make the "impossible triangle." Then color, levels, and hue/saturation layer masks were applied. (5 years and 2252 days ago)