I cant count how many times ive seen product photos of drugs sitting on a bathroom floor shot with the flash on and compressed to shit. Some vendors will take the time to acquire a decent camera and some lighting and make their stuff stick out. Others will just steal good photography, but a reverse image lookup will spot them. Once in a while a vendor will create something really cool.
What about a end of the year contest to promote good photography? Like a DNM sticky that will hang for a month and give them exposure.
Vendors who take the time to do good product photography reap the benefit of more sales. I don't think any other reward is necessary.