I’ve seen this question a couple of times here and no one seemed to have a really satisfactioning answer. I stumbled on accident on it.
Q: When one attaches a file (often tiff) to a channel (color, diffuse, etc) the preview stays black and in the viewports the objects are transparently viewed. What’s going on.
A: It seems this only happens when one stores the image (in photoshop) in indexed color mode. Tiff (and some other formats) can handle indexed colors, but apparently maya can’t handle the indexing function of tiff. Solution: open the file in Photoshop, convert to RGB-mode and store again. Reload the image in Maya and off you go.
 this problem can interfere with the fact that the bevel tool sometimes results in a loss of textures resulting in the object to be grey.