Veto ink is just a name of one of dye-based inks which generally much stronger than pigment-based inks and can produce much more color of a given density per unit of mass. However, because dyes are dissolved in the liquid phase, they have a tendency to soak into paper, thus making the ink less efficient and also potentially allowing the ink to bleed at the edges of an image. The significance of the photo paper in the whole technique is of that one it's waxy surface does not allow the ink to come into effect instantly everywhere,so the thin lines are mixed with the "blurpy" lines in one picture.

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Note added at 1 day6 hrs (2009-03-10 17:08:46 GMT) Post-grading
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Thank You ! Veto ink has another (obsolete nowdays) name which is lost among dozens of names of new materials appearing every year:ЧЕРНИЛА. I've tried to find information which I vaguely remember - the reason why it is called "Veto" because it is absolutely impossible to wash it out if you stained your fingers, your clothes, even your table.