Thanks for the replies. I'm finally beginning to understand the mechanics of the process.
This seems to confirm my wish for a visual guide to what file is for what UI component, although any variation in naming files would be a problem.
The flashing state comment was helpful. When I mouseover the taskbar graphic it does change from light gray to light yellow, so that would be the two states when doing this on Windows taskbar. I see 10 taskbar-related graphics files in this skin, including taskbar_buttons.tga, and none refer to flashing.
Yes, SKS is very complex and it makes my respect for those who have done these skins even greater. I'm just going to have to live with many failed attempts before figuring out what actually works and why.