How to add a local image in my firefox entension?

Question

I am developing a firefox extension and create a table and in it add a image , and I create a image with : var _img = document.createElementNS("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul", "xul:image");

and then I found I couldn't set its attribute "src" with a local image just like use its Url: chrome:\...., so I have to locate it in a web url:http:\ , but a problem will arise, when the http:\ couldn't be visted, How to do? if can I set the attribute of a image with a local url? Thank you very much!