Double-Click is a shortcut to drill down to original data, in this case the attached image.
Thumb Image is an optional field, starting on Item card, where you can Drop a picture. Big Business will automatically attach the original picture in Documents folder (Docs). Thumb Image will create a reduced-size version of the picture to display on Item card.
Double-click to open the picture. The picture also appears in Docs folder where you can Open, Edit, or click Send to make it available to email with Quote, Order, Invoice.