Even with practice, images can be fleeting anyway. I would suggest having a small pen/cil and note pad and doodling the images as you remember them, They do not have to be crafted works of art, but they do have to be enough to jog your memory of the vision itself when you look at them.
A technique I have that helps me remember visions is by imagining that my eyes are a camera. I'll look at a scene for a couple of seconds and then "see" and "hear" a shutter go off and then imagine you have a photograph of the scene. Looking away from the scene or closing your eyes, recall the photograph and identify as many details as you can, comparing your mental image to the actual scene.
One thing that will help train your visual memory is by recalling dreams. Keeping a dream journal (or a separate note pad) by your bed when you wake up, record every detail that flows to your mind. You don't have to remember the entire dream right away, but try to stay away from concenrtrating too hard. I find this, in myself and other people, to be creating a mental block as it sets up your mind for failure in recalling a memory. This will help you retain fleeting memories better as time goes on.
Once you are profficient enough in dream recalling and in mentally retaining photographs, try out dream reentry. This technique is done upon entering trance as it calls up a dream for you to either relive or delve into deeper upon being able to be conscious while "being in the dream" (some believe that this is purely psychological while others believe that you are entering a dream world, which is a subtle or spiritual realm similar to or in astral planes). This is by no means required, but it is a practice I advocate for as I work heavily with dreams and feel others could benefit from it if they wished to practice it as well.