Have you ever heard the film production saying, "every frame a painting"? It means that the cinematography of a film would ideally be good to the point where a person could pause the movie on any frame and find an artistically stimulating and memorable image. I'm sure if you take a jog through your mind palace, you'll find gilded frames holding mental pictures from movies you like.
An important thing to keep in mind is that the rules of media tend to overlap, and memorable imagery is undoubtedly no exception. The human experience tends to be very visual; the human imagination is naturally no different. When you were told how "totally tubular" and "rad to the radioactive meltdown" reading was as a kid, you weren't told you can smell whole news worlds, were you? You can spend a day pouring over psychology textbooks if you want to check, but if you ask me—the best way to create a memorable reader experience is to—
Write. Great. Imagery.
Now that we've gotten into why it's important to create good imagery let's get into some ways you can do it.
Forming the Image mentally
You can do this before you've written your scene or after—while in the editing process.
Let's say you're writing a crucial scene; a central character dies, the landscape changes drastically, or a devastating event sets the story into motion. You want the scene to end with a memorable mental image. The key to doing so is simple. What would you find cool? In all likelihood, whatever you find amazing, others will find it just as awesome.
Put details where you want the Reader to focus
Like with a photograph, the focus of your imagery is defined by the level of detail you utilize. Background facts of the environment should be general, while the focal point of your description should be vivid.
For example, say you want to describe a dragon sitting on the peak of a mountain staring down at the main character. You can describe the trees, the jagged texture of the rock, and the deep blue sky, but you shouldn't get lost in doing so; your focus is the dragon; thus, it needs the brunt of your words.
Pay attention to the Order of your Descriptions
While detail is essential in communicating importance, the order in which an image is described can be just as important in many cases.
Let's get back to the dragon. Do you notice how I mentioned the least important factors of the image first? The existence of trees, the mountain, and the sky are important in creating the framing of your specific vision, but they aren't the centerpiece. Ending your paragraph talking about the trees instead of the fire-breathing murder monster will only confuse your reader, muddling whatever you're trying to communicate. The dragon is the threat in this scenario, it's what the main character is most concerned with, and thus it's what the reader should be most concerned with.
This is also important on the micro-scale. Following the advice of the previous segment, we would ensure that the dragon benefits the most from your paragraph's word economy. This, however, doesn't mean you should list random facts about the dragon's appearance without any order. Listing the details of your most important factor in terms of their own importance signals to the reader what they should put the most effort into imagining.
The dragon is a clear threat, so its most minor threatening aspects should be mentioned first, like size and color, and as you go on, you move to things like teeth and claws. If you ask me, I would end the description with the eyes. The eyes are the best indicator of the creature's intent. They let you know where the creature's focus is; if it's staring at you, you are likely in danger.
Incorporate the Tactics of other Media
If you've paid close attention, you've likely noticed my comparisons between cinema and photography. I said earlier that all the different forms of media overlap, and that's no lie. All media concerns itself with stimulating the human interest in some form or another, meaning that all media have tricks and tips that can improve your writing when adapted to prose.
For example, one wouldn't think of color theory as having a place in writing, but further thought can uncover that it has much to offer.
The dragon is sitting at the mountain's peak, which means that the blue sky would be the creature's background from our point of view. Taking the complementary color of blue (yellow) and describing the beast's eyes as such would allow the most critical object's most important feature to pop in its environment. This description creates a striking mental image that gives it a place in your reader's memory.
I hope you've gained a lot from what I've had to say. I hope to create another addition sooner than this one. With any luck, it will be so!