New answers tagged effects
You can use an online tool like this triangulation tool or this Delaunay triangulations tool with images that you upload.
I think that the closest are layers with different layer modes and layer masks, possibly combined with a clever layer tree structure. In the future, GIMP will probably allow much more non-destructive editing when GEGL will be used in more areas.
The Oil Paint filter mentioned in Scott's answer was removed from Photoshop in version CC-2014. For those using Windows, Adobe has recommended using the GREYCstoration plugin: Oil Paint is a very cool filter, but relied on a very large and complex code library used for an outdated technology (Pixel Bender) that is no longer supported. ...
How about this? bring your duplicated shape to the front. add black-white gradient to the shape. change blending mode to soft light.
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