I failed in doing something very similar. Could you explain how you split the image up into triangles?
The "Analyze image" uses JSFeat to apply a computer vision algorithm (feature point detection) to add some "interesting" points based on the image.
http://bl.ocks.org/mbostock/4341156
Then for each polygon, I take its points (center point or vertices or an arbitrary point) and find the corresponding pixel on the image. Then I extract the rgb color, and use that to fill the polygon (as solid color or gradient).