Best way of segmenting veins from arm?
So one good step to enhance the vein-like structures is coherence enhancing diffusion:


  Weickert, Joachim. "Coherence-enhancing diffusion filtering." International Journal of Computer Vision 31.2-3 (1999): 111-127.


So I first apply this algorithm to your image, aggressively. The next step is to identify the curvilinear structures, which would in this case extract the veins. I already explained how to do this robustly in a previous post. So please check this.

The result is the following:


One could apply all the desired noise cleaning to this image. Note that the edges appear very weak, so it is better to tune the parameters, probably even more than I did.