After 15 years doing UI development, there\'s very little I look at and think, \"how on earth do I do that.\" This is one of those times.
A graphic designer has sold
I'd have to recommend RaphaelJS (see http://raphaeljs.com/). It is IE7 compatible, and you can do the triangles just fine: you need to do the math, but quite possible.
EDIT: look at http://www.chittram.com/editor.jsp for a quick sample of some of the shapes that can be done. That site is an interactive editor but the core capabilities you need are demonstrated.