Did you click on what I boxed in red?
And did you read what I boxed in blue? It says they'll resize it for you.
It also reads that the valid file extensions for this are .gif and .jpg (or .jpeg)
EDIT: It would help if I also displayed the functionality of the tool by attaching the image I'm referring to, wouldn't it?
Unfortunately it does not seem to be present when one is editing their post, so you have to use it on your initial posting...unless there's a way Wayne can incorporate that into edits as well. Anyways, see below: