I love the fact that this site supports $\LaTeX$, but some things are more easily explained with a free-form drawing. Many people I know have a drawing tablet, and many javascript libraries to allow such input are available freely.
And of course, making a drawing in your favorite MS Paint substitute and uploading it /works/, but I doubt anyone would like to do it that way. Integrating free-form drawing input into this website would allow many mathematicians to express their ideas more freely.
Edit: I would imagine that in the edit box, alongside the "Insert image" button, there would be an "Start sketching" button, which would either open a new window or get some space in the page itself, where with a minimal set of buttons you could make an easy sketch of your thoughts. Nothing fancy, just click to draw, no standard forms like squares or circles, no airbrush tools or layers or filters or effects, just a pen, and maybe colors.