Curt, Instant ThumbView is unreliable because it does not always render the thumbnail. For instance, it can be something as simple as the file name that creates this behavior (not an unusual filename either).
Personally, I'll just use thumbnail view or when that's not possible, I have Xentient Thumbnails
installed, which basically creates the file icon from the actual image.
Thanks Crono, I was unaware that the XnView team had a context menu viewer...I will check this out. I currently use FirmTools ShellExtension
If anyone needs to have thumbnails set as the default view in Open dialogs, you can use OpenWide
(add programs to an ini file to have separate views for each application), and Direct Folders
(HORRIBLE, if not non-existent customer support and unstable behaviour...I see the guy has had time to release a new app, but not respond to my support request of many moons ago - DO NOT buy anything from this guy, support your friend Nighted, ok?