Buying/renting additional userpics is not going to happen, because userpics are the single most expensive paid account feature - the cost of extra userpics would have to be the same as the cost of a paid account.

I think (though I may be remembering wrongly here) things like buying 5 email post credits aren't going to be done because it's too fiddly to keep track of which features you're paid for - free/paid/premium is much more straightforward, and allows you to keep track easily of what features you have. However, some of the discussion on dw_biz may be of interest, as they tend to go into these kind of questions in more detail.