I can offer two explanations:
1) This story takes place in 1984 and at that point, Heath Ledger was 5 years old and not a movie star or world wide heart throb! So, looking at a picture, it had to be someone who was more contemporary in that time, and Steve McQueen would make sense.
2) "green-eyed" refers to being jealous. In English, we have an expression about the "green eyed monster" in which the monster comes out when someone is jealous. So I think in this case, Jack is referring to Ennis being jealous, not the actual color of his eyes.
Leslie