I hate to be cynical, but....
These people mostly can't read....
Stickers in the wheel well would be too dark to see, and would be ignored.
Photos? These people don't read newspapers.
Put photos up at the airports, and on the fences? They would be torn down in no time by the intermediaries in this business, and by the security guards, that are getting paid for looking the other way, knowing full well there will never be any come-back from their clients.
I wish I had an answer.... barbed wire? Anti-bird netting in the wheel well? A campaign with leaflets in the right places with two photos... "don't try this" / "or you'll end up like this" ?
And to be pragmatic...
Hundreds die in small boats trying to get to the Canaries, or across the Mediterranean.
Let's put our efforts in trying to solve that first......