So this is probably OK for now, but doing this with image sources seems pretty weird to me.  If you really want to do this with an image it's better to have a single image and change the background position so you don't have to deal with loading two separate image files.

However, why aren't we using  `icon-angle-up`/`icon-angle-down` in our icon set? That deals with retina display issues, adjusting colours in the future, and makes this toggling stuff super easy.