Since the screenshot contest only allows stuff available on Steam and no mods, does DTG actually notify you that you've been disqualified? I've seen lots of screenshots submitted to the contest from people that didn't know it had to be official content, and although users alerted them about this, DTG never said anything. Do they message the user privately or just never tell them that they can't win?
DTG won't tell you that you've been disqualified. They'll simply pick a screenshot that is within the rules and move on. I've never seen an instance of someone being actively told by DTG that their screenshot wasn't suitable, with the exception of the American route (I forget which one) which can't be used due to copyright/licencing issues.
Is this about IronBladder's hand-drawn masterpiece? Because they kind of did say it wasn't eligible. That said: I'd guess the most efficient way of working would be to pick a favourite then check it's eligible, rather than check everyone's then pick. I imagine you might get a ping if they noticed, but unless it's a potential winner I doubt they're examined in detail.