PaintDotNet is a freeware general-purpose raster graphics editor programme. I’ve been using it for years, and I’m going to attempt to help @Fruitcake get going on it.
Perhaps he could tell us (briefly) what he’s got in mind.
Other can ask questions, give input. We’ll see how it goes. 
I have written a novel and am trying to get it published, so far unsuccessfully. One option is to self publish it as an E-book. If I choose that option then I would like to produce a “front cover” for it.
It’s a Victorian coming of age romance, and begins with a sixteen-year-old lad rescuing an eleven year-old girl from bullies, then taking her home on a horse. My idea is to depict that particular “scene” of a girl and a lad riding double on a horse in the countryside.
I couldn’t find anything suitable on the web, so came up with the idea of starting with a photo of my grandad on his warhorse during WW1, then superimposing a second rider so they appear to be riding double.
I want to “draw round” (copy) my grandad from the original image and move him backwards, then superimpose my face from a photo’ of when I was about seventeen onto his. Clothing wise it just needs a bit of colourising, but otherwise what he is wearing pretty much fits the story.
I then want to place another rider in front of him, which could be done by pasting a shorter version of the same one again, and superimpose a photo’ of my cousin’s face from when she was about twelve onto the “front” rider.
Complications are that my photo’ shows me wearing clunky NHS specs’ with shadows and distortions from the lenses, so getting rid of the glasses is a sort of separate project.
The second rider’s arm needs to be bent up so it looks like he has his hand on the girl’s waist, and the girl’s clothes need to be changed into a full length Victorian “country-girl” dress jobby.
Clothing colours also need to be changed, and the second rider needs to lose the stirrups.
The main problem I have had is with setting the layers and switching between them. I keep losing stuff I have parked in separate layers, or the layer ends up on top of, instead of underneath another part of the photo’ I want it to sit behind. When I do get it “behind” what I want, I can’t then paste another layer on top of the top layer because it keeps being set underneath.
I’ve watched no end of tutorials, for several different programmes, but the presenters either speak or move too fast, or what they show on the video is not what I see on my screen.
Since d00d has suggested Paint.NET, then that is what I will stick with.
First, this is my base picture.
This is one colourised using Lunapic has sharpened the edges so I can see them more easily, and may or may not be better for me to use as a starting point.
This is a (very) rough idea of what I am trying to achieve.
The first problem I have is that I have a hand tremor which makes drawing freehand on a pooter, or drawing around Part of) an image difficult.
These are the two faces I want to superimpose on the riders.
You see what I mean about the glasses and distortions they cause.
What would be best to start with; colourising the photo, or reproducing my grandad as two separate riders?
Hi … so I’m confused,
How did you get this far and what happened to the colour!
Are you now wanting to remove the background? Are you wanting different images throughout the book using those same two heads?
Have you thought about using illustrations/sketch type drawings?
A bare bones version of this kind of thing with white vignette surround?
The image with the “vibrant” colours is what I managed before I asked for help. It’s merely to demonstrate the idea, the concept, of what I am trying to achieve.
What I want to do is produce a single image for a book cover consisting of two young people dressed in Victorian period clothing, riding the same horse, in colour, ideally with a satin finish as you would get on a physical book.
I like the sketch drawing version idea, but I would like to see a traditional version first as that was my original thought.
The thing I’m have most problems with is understanding how to get layers to work. I can’t work out how to drop a selected area into one then place it above or below another layer without losing it amongst other things. I also can’t work out how to accurately draw round areas I want to select.
It might be better to use an AI image generator such as Adobe Firefly. Just type in text the sort of image you want (for example: victorian era couple riding the same horse whole picture)
Just make sure you can use the image for commercial use.
Just keep pressing the ‘Generate’ button until you get the sort of image you want. Art or Photographic modes can be chosen. I’d suggest ‘Art’.
Thanks Mart. I hadn’t thought of using AI.
Do you have a literary agent Phil? I ask because without one, you’ll be on a hiding to nothing. Also write under a nom de plume.
I’m not trying to put you off, but there are many to that publishing tree and hence the need for an agent.
I’m working my way through a list of Literary Agents provided by membership of an outfit called Jericho Writers. Some agents don’t respond, some just reply along the lines of, “it’s not what we are looking for,” but occasionally I get encouragement despite a rejection along the lines of, it’s enjoyable, or, it’s well written, or, you have brought the period to life.
I already have a nom de plume. Madame Fruit-de-Cake of course.
I’m not doing it for fame and fortune, but because a few people have suggested it’s a book worth reading, so I’m giving it a go. As I said, I’m currently looking into self-publishing, and learning how to photo-edit will be useful for other stuff even if I am unsuccessful.
I was aware of all the other elements involved, but I appreciate all the advice I can get. Thanks.
I think @mart has got the answer 
I tried looking for copyright free images, and came up with something like this from https://www.freepik.com/
which of course is not suitable but might look good on a book cover … the watercolour aspect. I was looking for vector, didn’t think to look for AI. I’m normally use PDN for my own images.
