Right now images do not support the invisibility var (you can set it but it doesn't function). It would be awesome if they did.
I know that there are very few situations in which this function would be useful, but those scenarios do exist. In addition to using images to only show things to certain players we also work with a lot of different see_invisible layering in NEStalgia. Adding a lot of extra checks to add/remove images from client.images would be too clunky for what we're dealing with.