I haven't build even my cabinet yet

but this is a technique I used in my kid's room, and there's no reason why it would not work elsewhere

Get the images you want, and modify them in a photo package / image editor. I used the same technique for the fish as well as for Ariel. I found an image of a mural (something airbrushed) on the net, which I resized and printed out in the appropriate size. I had to tape a few sheets together to get the proper format. As said before, same thing for the fish.
This is the moment to do some modifying, I decided to remove the 'crown' and parts of the dress of the original image, and change the part of the crown in a sort of shell / hairpin. If you look at the 'taped printout' you can see the things I left out. (Yes, those are post-its, great for NOT damaging the wallpaper when doing things like this

)
Next step was 'fixing' the whole package to the surface (in this case the wall) and then I simply 'traced' all lines with a pen, applying some pressure to cause the surface below to indent.
After that I used a small black marker and drew those lines more visible, this simplifies painting. The plants, the blue fish, second yellow fish, rock stuff and 'shiny whatever at the bottom' were freehand.
Once I drew all lines in, I took a few spraycans to allow some 'shine' or 'wave' effects which are hard to do with brushes.
Then, finally, came the hard part. Using the print out, and a bunch of cans of paint, I set to work. I mixed the paint 'on the fly' and made sure it never got too bland.
Once done, I touched up again using the spraycan to give a certain shine in some areas, and done.
Pictures attached (looks a lot better in real life though, the flash in my Canon D350 sometimes brightens up things too much, the one with just the fish is the real colour). Be aware this is very time consuming (the mural took me two days). You may want to do it on some other material, then scan it and get it printed, that will save you some time when you have to redo it

I've actually been thinking about taking a few more high res pictures and having it printed on transparent vinyl. Would look great on a light blue cab.
(Oh, just a note: I used regular paint for this, all left overs, and an ice cube box to mix it up. It's easier with more suitable paint, but what the heck, this did cost nothing except some time.)