Mike,
Tony Canadeo played for the Packers from 1941 - 1952, except 1945, when he did not play.
Don Hutson played from 1935 - 1945.
Curly Lambeau was the Packers coach from 1921 - 1953.
So, the picture would be anywhere from 1941 - 1944.
According to the Packers' 2015 media guide, three players wore #51 from 1941 - 1944.
Ed Frutig wore #51 in 1941. He was a 6' 1", 190 lb end.
John Stonebraker wore #51 in 1942. He was a 6' 3", 200 lb end.
Irv Comp wore #51 from 1943 - 1949. He was a 6' 2", 204 lb QB, RB and DB.
Here's a picture of Frutig from wikipedia:
Here's a picture of Stonebraker in 1939 while he was at USC:
And here's an Exhibit card of Irv Comp:
I also found another copy of the picture you posted, as well as this one, on ebay, where #51 is identified as Irv Comp.
Steve