Hey, if I'm the mailman, I can walk all the way up to your front door. Slipping the picture inside would be expected. My mailman leaves stuff in my mudroom to keep it out of the rain & dirt (sometimes bears).
Granted, that doesn't apply to people who don't have a customary reason for being there. Police officer can chase a suspect without trespassing, mailman can come up to your door and slip stuff inside, building inspector can come inside for inspections as they wish, etc..
A person who has contacted the local police station and registered as a solicitor is welcome to place flyers on your doorknob. There is no trespass in that instance. Many moons ago, I worked as a canvasser for a political organization, and walking onto private property was a big part of the job--but the entire team generally needed to register with the local police.