Many if not most of the tutorial makers out there don’t know how it works either.
You probably know much of this but it’s difficult to explain in part and I’m posting for the lurkers and searchers as well. Some terminology and minor details may be wrong of course as usual 'cos Blender is a beast. Any corrections or more tips out there post them up please.
Faces get images mapped to them when they are selected and you pull up an image in the image editor. You can map multiple images to different faces on the object. If you go into face select mode and select a face on the model, the image in the image editor will switch to whatever image is mapped to that face or go blank if no image assigned.
This works differently when sync mode is enabled. Then all faces are mapped to whatever image you load, no matter what faces you have selected.
You can see this mapping in 3Dviewer multitexture view / texture mode. You can only see this mapping in a render if you apply a material and activate Material > Options > Face Textures (and no textures are loaded in the texture panels).
In 3Dview GLSL textured mode you only see textures loaded through the texture panels.
So anyway, this is why images sometimes won’t load… you have to have at least one face selected for it to map to. Or you’ll have an image loaded in viewer but it mysteriously switches on you when you select some faces on the object (or select object in paint mode? -I don’t remember). Anyway, it’s because that image you want has not been mapped to any faces but a different image has been.
Or why you can get an image up (by going into paint mode or however -I don’t remember) but it not show up on the object when you paint on the texture image -because that image isn’t mapped to any faces on the object.
Or why you can paint on the object but it not show up on the texture image -because the faces you are painting haven’t been mapped to that particular image. You don’t see anything happening because you are painting to nothing, a different image, or even multiple images at the same time, none of which are what you are looking at in the viewer.
Get into face select mode so you can see what images are mapped to what faces (it doesn’t work in vertex or edge mode). That is the main tip. That and edit mode selecting faces (or going sync mode) before you load up the image you want to work with so it gets mapped to those faces.
This edit mode mapping of selected faces (or all faces if in sync mode) to images, and not knowing to get into face select mode to see what is mapped to what, is the root of most complaints about the UV/Image editor being buggy. It is not buggy. Maybe quirky but not buggy. Spread the word.
– Textures are loaded in texture panel. Look for brush icon at top of texture panel.
Hope this helps.
BTW in the latest Blender revision I just downloaded I’ve noticed a change… when you unwrap and load an image it won’t load into the viewer until you reselect a face on the model. I see more confusion ahead for many if it stays like that. EDIT: Or maybe not. Might have just been me… will have to test again later.