An astronomy forum and selling site that I was on became a "pay to post" site a few years ago, and it dwindled from hundreds to just a handful of users after the announcement, and it was only $12 a year. Formerly it was run on donations, which I did a few. My participation in that forum was already declining, so I chose not to continue. There was another free site that started up a few years before that, and most users went there.
Making a forum a pay site just to post is a certain path to kill a forum.
The problem is, a free forum isn't free at all. Somebody, somewhere, is footing the bill. And having the space to store photos costs money.
Consider this: when I was younger I was heavily involved in hot rodding. I subscribed to Car Craft, Super Chevy, and Popular Hot Rodding magazines. To the tune of approx $100 a year if I recall. Now, a subscription to Car Craft is twelve bucks a year. And, that's for a dozen issues, not six!
And is it really "pay just to post"? Someone else mentioned the comraderie and interaction as being draws also, which I happen to agree with. I don't recall the last time I asked a pond or fish question here. But I've sure tried to answer a few, so exactly what benefit have I reaped by hanging around? I have friends here, that's why. I feel like my days of NEEDING the forum are probably behind me, these days I'm here just as a social event. Now the uncomfortable question: shouldn't those who are utilizing the forum as a source of info that they would otherwise have to pay for, incur some debt in this regard? Should I pay for the privilege of answering questions, rather than receiving answers, myself?
I realize it's blunt, but there are times when bluntness is needed.