Note that this path have to be available in the machine where oned is running.
Otherwise, you have to find a way to move that file there. You can also provide an http url as the path of the image and OpenNebula will download it. Therefore, you can deploy an HTTP server in your machine and provide the url of the file.
You can upload images from Sunstone but I would not recommend it for big files.
I would expose the image in an HTTP server and provide an url as the path of the Image.