A thumbnail is a small version of a larger graphic file. In a Windows 98 Explorer view, you can easily turn thumbnail viewing on so that you can see a small representation of the picture contained in the file. Here's how you do it:
1 - Right click on any Windows 98 Explorer folder
2 - Select Properties
3 - Choose "Enable Thumbnail View"
4 - When you open the folder, click the View menu, and choose "Thumbnail View"
5 - To undo your operation, simply select any other option from the View menu.