I don’t think there is such a provision to bring in photos from machine into microflow but you can use image uploader in a page by keeping it in some data container. :)
You are using the gallery widget to show a list of images, based on an specialization of System.Image?
If this is a correct assumption, then just put an input parameter in your nano- microflow. This nano- microflow should be connected to an clickable element in your gallery. Either inside an object or as onclick action.