In thumbnail view you can have a picture for a folder. If you have a lot of pictures in the folder then those pictures will be shown in the thumbnail view of the folder.
However if you want to have a picture as a the thumbnail for a folder in the thumbnail view , do the following:
Right-Click the folder and select properties. Go to the customize tab.
Click on to the choose picture button. Just browse for the image and click on to the open button.Here after when you view the folder in the thumbnail view, the selected picture will appear.