I know casual copyright infringement is the norm on the internet (eg, distributing images from god knows where), and maybe that's okay, but you've certainly crossed an ethical line when you've started selling it without permission.
The troll face would have NEVER become popular if it was not free to distribute in the first place. It's not this guy who made it popular by himself, it's the 4chan and subsequent communities who did - in that sense, what it became does not belong to him at all.
The value of the image is 99.9% associations it aquired thanks to the "copyright is bad" culture using it under assumption that it is public domain.
Changing it now is bad style, and it sets bad example for others that may take this further. I know slippery slope isn't a very strong argument, but it is still valid concern.
I wonder if 4chan should adopt GPL-like licencing.
> Image Credit: Adam Sandler (Unknown, please let me know!)
Many of the uses you describe are covered under Fair Use (and then therefore are not infringement).