Invitation Watermark

This is what a business in Australia was looking for in their invitation design

My company issues vouchers (like the one I attached) for customers to redeem at the respective theme parks or attractions.

I would like some sort of background watermarks/pictures added to the voucher to:
1. Make the voucher look more lively
2. Discourage unscrupulous people from thinking they can just edit the voucher and claim more items than they had paid for (such as changing the number of adults from 4 to 5).

I realize that most professional graphic artists will be able to replicate the watermark. This is really just to make the voucher look a bit nicer and add a "discouragement".

I have attached a sample. The wavy lines occupy the most important section. The watermark should at least cover that part. Read more