This is what I made:
The middle figure was more or less from one of the anatomy texts, and the mermaid was slightly modified. And I think I'm going to have to experiment with using a different lens for photographing paintings, because the original really is better than this, honestly!
things I learned : try making an armature for a figure drawing with lines that fade away into a triangle, and find where the weight of a figure lies. Also, my burnt sienna/titanium white mix for flesh tends to come out too muddy, he suggested yellow ochre, vermilion / cadmium red and titanium white, with some phthalo green for shading. I don't have any yellow ochre, so I've ordered some, but you can see a bit of the green in this, I think.
Also, he thinks I'm just trying to paint too small, and should try making some bigger images so that it's less vital to get every speck of paint in exactly the right spot. I had just bought some cheap canvases from Lidl, so I can try it, though the problem with canvases is finding somewhere to put them, I already have a mountain of paintings on card that I can't quite bring myself to chuck out. Perhaps I will just bite the bullet and overpaint them with something else.