Yul's image is taken from his role as the western hero Chris Adams in The Magnificent Seven. This sticker is clearly labelled with Fork's name.
Steve's image seems to be taken from his role as the American pilot Hilts, "The Cooler King", in The Great Escape. The sticker has Fork's name subtly written on it.
I later came across another great sticker by Fork in the White Street car park. It came in many hues and shows actor Alan Ford having "a moment" in his role as the gangster Brick Top in the film Snatch.
I took all the above photos in March 2014.
In case you are interested, here are links to a few clips on the 'Tube, showing these actors in character....
Yul Brynner as Chris (with Steve McQueen in the background):
Steve McQueen as Hilts:
Alan Ford as Brick Top: