I have wondered about this too. I think the answer, that they are so carefully not saying, is that it changes as needed. Around the time I first started BYC, it was said you had to have 10 posts. There was quite a bit of spamming going on then, so that probably had something to do with it because later I noticed people saying that the post count had to be even more than that. Nifty was very vague about it- Make a few posts and then try.
I get that it can change, but it just kills me that they can't just say, "Right now you have to have x posts - that could go up or down as needed" -- because, like I said, it can be really confusing when one person can't post photos and is told it may be due to post count and then you go to an introduction post and see the new member there having no trouble putting pictures in their first post to the site, which I have seen this all on the same day........silly me thinking asking would yield a straight answer.