Its pretty hard to tell unless you show some code. I notice here that the file icons are displayed when I change folders then the thumbnails are shown, so maybe it is a video card problem or a refresh problem. But maybe Nigel will have some other ideas.
Funny thing is, I can run the FolderView demo and the images show, just not in my application. I copied just about everything from the FolderView in terms of code and I set the Folder property to the desired folder.
I know now why this is so. My application is a 64-bit and compiling the FolderView to 64-bit gives the same issue as my application. So, it looks like a 64-bit issue of ImageEn.
When I compile the demo as Win64 I see the thumbnails. I'll bet you do not have ielib64.dll in the exe's folder. When building any ImageEn project as 64bit the dll must be in the applications exe folder. I renamed the dll so imageen does not find it and I get the same results as you. I only get thumbnails for ico and GIF... everything eles appears as the file icon.