... edit ...
So, please tell me - are higs res textures included in daz sdutio/poser distributable, or they must be purchased separately? And, generally , how one could use them?
No, you can not redistribute high OR low res textures from either program.
Yes you can purchase skin textures separately from Daz3d.com and other places.
There are some legally free texture sets here and there; like over at sharecg.com and in the free section at Renderosity.
Generally speaking ... one can use only legally obtained textures and models to create a scene ... the scene is rendered out to an image file like a .jpg or .png. Said artwork can then be redistributed for free or for sale as one pleases according to the conditions set out by the providers of all the content in one’s artwork. Not all freeware can be used for commercial work for eg.
Here are a few in the Gallery: http://www.daz3d.com/i/galleries/
If you want to create character skin sets yourself, there are several “merchant resources” available for purchase with which to work from. Again read the conditions of use. For eg. not all may be used for creating free skins or character sets.
Edit: here is one of the several available skin texture sets uploaded at sharecg.com