Thumbnail storage explained
CaptureWiz automatically keeps backups of every capture and displays them as thumbnails in the main window. By default, only the last five days are stored, but you can change the number by clicking File / Options / Main, and setting Number days images stored. You can set the number from zero (no storage) to "no limit".

You don’t have to worry about stored images taking lots of memory. CaptureWiz only stores small thumbnails of your captures in memory (not the full images), and it only stores thumbnails for visible items (not all of them). On the hard disk, CaptureWiz stores images as highly compressed, but 100% accurate, PNG files.