Having investigated the Twine engine code, I've found that the official passage syntax documentation(*) is incomplete. Here, then, is what I believe to be a complete list of Twine markup syntax. Almost all of it is derived from TiddlyWiki, and as such, some parts are not necessarily relevant to the typical Twine author. Nevertheless, they remain available.

http://www.glorioustrainwrecks.com/files/Twine1.3.5.MarkupSyntax_0.html

Updated 27/3/13: Added more inline image syntax examples.