Hmm, think you might be right about lust. Think I might go for pride or vanity ... or sloth ... or perhaps gluttony.
Oh poo.
The original seven sins were pride, sloth, hypocrisy, wrath, avarice, hatred and gluttony which were depicted in 13th century art before the church decided what the actual list was. I don't know if perhaps that broadens the spectrum or the challenger wants us to stick to the official list.
Just found the punishment list too (I studied religion as a degree hence the annoying constant posts).
Pride - broken on a wheel
Envy - put in freezing water
Anger - dismemberment
Sloth - thrown in a snake pit
Greed - boiled in cauldrons of oil
Gluttony - force fed rats, toads and snakes
Lust - smothered in fire and brimstone.
Hope this helps (that's if anyone needs it of course).
I think I might actually go for a 13th century art style. It will hide my not-brilliant drawing skills.