Have you looked into the upcoming
5.1.12 pre-release version?
It has an improved image editor, with selectable stroke width, color and transparency. Just create a new tiddler and change the type at the bottom dropdown into Image/PNG or JPEG
It saves them as standard images, not sure if that's the level of editability you are looking for, but you should be able to edit them on your computer with any standard image editor, or even directly inside Tiddlywiki