Wow - you really dug into this!
First of all, I don't get any thumbs.db, except for when I had ACDSee installed, so I guess there are some apps that still use them, but Vista doesn't use them on it's own.
Secondly, I found a desktop.ini like that in the folder 'pictures' and it runs like this:
[.ShellClassInfo]
LocalizedResourceName=Pictures
InfoTip=@%SystemRoot%\system32\shell32.dll,-12688
IconResource=%SystemRoot%\system32\imageres.dll,-113
IconFile=%SystemRoot%\system32\shell32.dll
IconIndex=-236
[LocalizedFileNames]
Sample Pictures.lnk=@%SystemRoot%\system32\shell32.dll,-21805
But the funny thing is I never use the picturesfolder. Everything is in my Documentsfolder and in there there is no desktop.ini, but I do have it set to viewing images and videos (not to all files), so I have large icons and they give a preview of one or more files inside the subfolders.
Edit: I tested it to be sure, so I gave a folder a specific image to use as content for the icon, but there doesn't appear a desktop.ini. Not in the folder that contains the folder or in the subfolder itself.
However, I think for burning purposes I can use the desktop.ini system by placing a hidden desktop.ini and an icon of my choice in the folder - that should work. Maybe not the same as a folder with a specific content, but I can always create an icon of a folder with that content myself (IcoFX is great - http://icofx.ro/ ). I have about 10 folders in all in mind, so that's still within working range ;-)
I won't do it that way on my downloaded apps folders - that would keep me busy for the next two years LOL
However now you got me curious about the content of that ini file.
When changing the icon on a file it also uses paths like %SystemRoot%\system32\shell32.dll instead of C:\Windows\System32\Shell32.dll
When browsing for an icon I can use set paths like that and it works fine.
Most systemicons are found in shell32.dll and imageres.dll
Other than that I can't provide input.
Goodnight!
Sarah