Yes, as you've seen there's quite a difference. And as Stemardue shows, the TEXTURE map is not meant as a bumpmap, that's why special greyscaled bumpmaps exist. And an ideal bumpmap isn't just a greyscaleversion of the texture map either :o) Part of the confusion is probably caused by Pro pack Mat poses, which attached the bumpmap - wrongly - in the gradient bump channel. Here's a comparision of the two bump channels. As it's clear, the gradient bump is almost invisible with the jpg plugged in.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.