Pigs breed as they're needed, empty pastures aren't considered a "need". A tradehouse constructing wagons would be considered a need, though.
That's a good point about the tilesheet guide, I'll try to remember to do that for the next patch.
Oh. Well I guess as of now wagons and wealth are the only uses for pigs so it doesn't really matter. I know pig trading is planned, but maybe we could eat them one day? Possibly around the morale update as a luxury food.
Pig meat will probably be one of the first luxury items.
