In the bar directly above the thumbnail listing is a little window that (by default) says 'Selected'. Press on the funnel button to its left and pick 'All Files', and all files including any .iso files should show.
Alternatively, you could go into Options | Preferences | File Types and create an entry for .ISO, so that it becomes part of the filetypes selected for display in the thumbnail listing.
.ISO is not an image format (in the photo or drawing sense) so TP won't be able to create thumbnails for it. Instead it will show the icon of whatever program is associated to the .ISO extension in Windows.