Hello.
I am starting with ZBrush, and a couple of things I have found is that if I save a history of files of my project along the way, all of which have the same texture, that if I close ZBrush and come back, the textures are no longer in the texture list of any of the files. If I have a series of textures that I have color-corrected in Photoshop and tested them out in the ZBrush file, if I close ZBrush and come back, I have no idea which version of the texture I used.
A related problem is that if I am working on one project, and then open an unrelated project, the texture list still contains the textures from the last project, not the one I just opened!