Detect and isolate veins in the palm?
Ridge filtering seems like a possible processing. I have not tried to clean up the non-veins ridges; perhaps there would not be need for that step in more constrained acquisition conditions.

In Mathematica:

img = ColorConvert[Import["http://i.stack.imgur.com/5og5Z.jpg"], "Grayscale"];
r = RidgeFilter[ColorNegate@img, 3];
Binarize[ImageAdjust@r, .03] // Thinning