Using a proprietary barcode format won't help if people post the barcode image itself (which they do in the example image provided). More work might be require

Using a proprietary barcode format won't help if people post the barcode image itself (which they do in the example image provided). More work might be required to 'unskew' perspective shifts to get it back looking like the original if the photo wasn't taken from directly in front of the printout, but barcode readers are generally designed to deal with that sort of skew anyway. I remember when the Xbox One came out last year a lot of people were posting ebay listings for digital download games in the form of scannable QR codes and posting those QR codes as proof of the item existing (since otherwise it is all digital). Not surprisingly people claimed those QR codes using the ebay images without ever buying the item.