If you wish to protect your thumbnail images shown in your Order Report/Invoice, then the best solution is to use the Order Report option "Add Copyright Stamp" to stamp each thumbnail image.

Note: This is Order Report Options window is shown from ProSelect v2015r1 and may differ in other versions.

For information about Stamps & Watermarks (and Setup and Applying Stamps), and "Order Report Options II" see the ProSelect Reference Manual (quick access under the Help menu in ProSelect). Again this may be different depending on the ProSelect version you are using.