Your question is interesting and you'll probably get many different suggestions for this since no one has discovered yet a unique formula to artistic "talent" and imagination. But there's interesting discussions on this. So this is my suggestion.
(I think) Talent is a myth, it's simply observations and constant practice applied at every opportunity that makes people look talented; that's a slow evolution that isn't always noticed, so it's hard to know exactly where it comes from and if there was even a beginning. And imagination is similar; it's by accumulating inspiring ideas that make your brain happy that probably makes you want to get more and more of it. So what doesn't exist, you create it in your own mind. In a way, a good artist is very boring... it's simple biology and biochemistry in the end.
These might be "meta" concepts, but I'm tempted to think all this has a lot to do with training your "spatial intelligence" and the good old "10,000 hr rule" before being able to master something (if you believe in these things).
So do like Hemingway (at least, his career, not his final act) and practice. It will help you develop a muscle memory somehow. Apparently, you already have drawing technique. Now it's time to work on the imagination part, not the technique.
If that may help, I remember reading some study on how humans end up having difficulties recognizing real human symmetry from a digital rendering the more they spent time with games and the digital world, and less with their fellow humans. Their perception got distorted and it kind of changed a bit their perception of reality. From memory, this had to do with biochemistry as well since there was a certain attachment to the environment they evolved in.
So I guess we could take from this that our knowledge of what things should look like is something that isn't permanent but malleable. You'll certainly learn more about movement if you simply sit in the park, watch people and draw them, rather than using pictures telling you how to do it. Using pictures is kind of pre-digested information to you and not something you can develop a connection with, at least not in this sense.
You mentioned you're already very skilled at doing photo-realistic drawings but have hard time drawing from your imagination... To me, it seems like you are a bit stuck in your 2D universe, in your Flatland, the same way the people in the study I mentioned above probably are. So my only suggestion for this is to go out and challenge yourself by leaving the 2D reproduction aside for a while. If you're used to reproducing flat images that have no movement, you're not practicing your memory skills and you might have to start there before being able to put on paper the images or scenes that you see in your head. The photos you use are like a crutch. Often we end up doing things a certain way because we're good at it and that feels comfortable and satisfying. But maybe you're ready to get to the next level and should just jump and do as much "live" reproduction" as possible without judging yourself too harshly while you're "evolving". The same way you did when you started drawing.
I cannot give you any "construction method" as my method is usually "just try and keep going"! (see Vandervert's approach) If you look at great artists such as Dali, you will clearly see their evolution, and how they developed their mastery, even for the abstract, in the same measure they kept working on what they found was beautiful to them. When you find something beautiful, you also want to keep looking at it to know its every sides and facets, and you might even notice it instantly everywhere in your environment. And you end up "knowing" it. So I guess you'll become a very good "imaginative" artist by drawing real things you find beautiful and merging them with your own concepts of perfection... and practicing it without the photo-crutch! You can't get that kind of connection or passion with a picture. What you're trying to do now is almost the same as learning a new software.
I do hope this won't look entirely like an opinion. There's certainly good studies on this and I can probably edit this answer to add some links if I don't get stopped by the many paywalls. There's certainly strategic content in this answer though!