I found this information at the Post office.... maybe you will find it interesting as well. I've condensed it a bit.
"This year, Canada Post marks the 100th anniversary of Yousuf Karsh's birth with a trio of exceptional stamps featuring three of his famous photographs."
It goes on to say...
"An international rate $1.60 stamp features the photograph that made Karsh famous.... taken in 1941 when British prime minister Winston Churchill visited Ottawa... the Churchill photograph was an obvious choice for the stamp."
For the US rate $.96 stamp they picked the tranquil portrait of Audrey Hepburn while a self-portrait of Karsh at work was chosen for the domestic $.52 stamp.
They go on sale on May 21.