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.