Albert Vickers emailed me the following information on the 9th of January 2008 as some background information to the photo -
The line up of mules you have on your site is as I said, a line of working mules. I took the photo in June 2007 in Sorocaba District of Sau Paulo Province Brasil. The guy in the photo is my friend, the young veterinarian and trainer Fernando Rolim.
An amazing photo!