Invitation Watermark

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This customer received 15 invitation designs from 10 designers. They chose this invitation design from Regina_T as the winning design.
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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.
Industry/Entity Type
Artists
Requirements
Must have
- The background watermark/picture must be in grayscale (in light gray), and should be distinguishable when printed. However, it should not overshadow the actual text.
The final image should be vector-based so I can scale it easily.
I used wavy lines in my sample. You don't have to follow that.