A good image would be a lamb bone, as inside the joint of meat itself would be the actual bone which gives the 'weapon' it's strength and weight. In the short story 'Lamb to the Slaughter' by Roald Dahl, the author presents a murder weapon which can actually be eaten! However, it is important to remember that the bone remains afterwards. It is probably this which makes the weapon effective - although the weight of extra water in a frozen joint would be considerable. So, I would go for the image of the bone itself. A joint of meat without a bone inside would not make a very good murder weapon - although we should always note the black humor of the police eating the meat!
While a scene with the police sitting at a well-set table, voraciously eating the leg of lamb all while they laugh and talk certainly exemplifies much of the story's plot, I would focus more on Mary Maloney's innocent appearance. You could draw a noticeably pregnant, 1950s-style housewife placing the leg of lamb in an oven and perhaps off to the side (through a doorway) feature part of her husband's body or all of it on the floor. It might be more dramatic if you just draw his outreached arm in a pool of blood. If you contrast Mary's apparent innocence with the suggestion of violence in another room, you will illustrate one of Dahl's themes.
You can either draw a picture of the woman smacking his husband's head with the piece of lamb, or you can draw the police eating the piece of lamb which the other officers are looking for the weapon, and put words on the image showing the irony in that picture.