I've seen several agents and publishing houses say things like, "No unsolicited MSS; query first with outline, sample chapters and SASE," in their submission guidelines without any separate instructions for picture books.
In the case of short-ish picture books (400-600 words) with no illustrations, the whole text fits on one or two pages. I can replace an outline with a brief summary, but then, of course have no chapters to include. Do I send any of the short manuscript, all of it, or nothing beyond a letter?
Here is an excellent opportunity for me to say that I do not know everything. If an agent/publisher represents/acquires picture books and yet gives no hints about submitting them, what does it mean? I have no fricking clue.
Other questions of this ilk include:
If two bears walk into a bar, what flavor of paella are they serving in Madrid?
If you're afraid of the dark, what the hell is a night rainbow?
If the unicorns steal my crack pipe again, I'm going to kick the crap out of them. Well? Where is it?