Note: images can be mounted read-write, edited to some extent and then written to the final medium (e.g. a flash drive) because there usually is a little bit of spare room on the image, but this is not a whole lot. For substantial edits that would increase the image size beyond what it already is, you pretty much need to create a new image including boot code and all that. In short: modifying an image is possible, but not necessarily easy.
