I think I may have fussed over and edited my first post so much that I whittled out the actual question.
Here is what I have:
A mesh body and head (one of the free ones, "RuthToo").
A skin, which I fully understand to be a set of three textures. I can see those textures by wearing the skin and selecting edit on its inventory listing.
An HUD from the makers of the body, which features illustrated buttons of several skins they have provided. Each button shows either head, upper or lower body. As per the instructions, this is the process for adding a SKIN*: "Right click on the HUD : Choose Edit : Click Edit Linked tab : Click on single button on HUD > Click Texture tab in Edit : Add Texture :" ... and that's where they stop. At this point, I understand that what I need is a TEXTURE item; specifically, I want one of the three TEXTURES that are inside the SKIN.
However, I cannot find any way to access those three images as texture objects. As far as the knowledge I have goes (which is of course quite limited til yet), it doesn't seem likely that they even exist as textures -- that is, as a datatype called by colorful-checkerboard icons in viewer panels -- unless and until some kind of process is applied to the skin object.
What I really need to know, then, is whether having the skin means I also "have" the textures, and if so, how can I access them? Or do I need to skip this and seek skin-patterned textures instead of shopping for skins?
* the instructions also say you can "drag and drop on button" to add skins, but that... that doesn't even... I mean, you can't drag skins "onto" an HUD button, right?