Tutorial on how to make a nice baby & daddy photo into a nice baby & daddy painting.
I tried a variant on this theme which requires a little less work but gives quite good results.
1) Duplicate layer
2) On Background layer use Layers->Colours->Posterize - choose 4-10 colours according to taste
3) On the other layer use Filter->Edge Detect Use Sobel
4) Invert the edge detect image
5) Desaturate it to give a line drawing, similar to the one shown in this tutorial
6) Set the opacity of the edge detect layer to around 30%
That gives a moderately good image for scenery.
You can tinker with colour options before posterization (eg equalize) and with the edge image (eg sharpen) for other types of image
Other edge detect algorithms are also worth a try (eg Laplace) - these produce black lines on a transparent field straight away