When the dev are so greedy that he lock stuff behind a paywall, smh.
Well, it's not so bad if it's just some extra side content or something like that, but one could argue that detailed artwork in an otherwise text-based game counts as "core content" that completely changes the game. And even extra side content shouldn't be the most prominent feature shown in the screenshots. Though I'd generally try to avoid using the word "greedy" on a piracy website. Glass houses and all that.
That being said...
Not sure yet, currently leaning towards making it a stretch goal.
If you mean having it be something that happens once you get enough patrons, I'd say that's not a good solution at all, and is even kind of backwards. Tying the availability of free content to the progress of the payments means that the people who are helping to make it happen don't benefit from it, and the people who need it aren't contributing to make it happen. Not to mention the fact that there's a significant chance that the goal will never be reached, so people will be left to just cross their fingers in the hopes that it might happen some day.
(Plus, wasn't the argument for allowing it in the preview images that it was considered to be "content the dev plans to add to the free version eventually"? I think holding the "free" version behind a different kind of paywall that may or may not ever be reached disqualifies it from that.)
Traditionally, what Patreon devs tend to do is make time-gated content for the free version that gets released early for Patrons, so for example the free version would only have CGs that are 1-2 months old. Though I am aware that promising anything for free will result in less people paying, there's a balance to be struck. Somewhere. Probably.