Thumbnail storage explained
CaptureWiz automatically keeps backups of every capture and displays them as thumbnails on the capture bar and viewer. By default, only the last seven days are stored. You can change where and how long images are stored by clicking Misc / Options / Main. Set the number of days 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.