What do platforms like Shutterstock, 123rf, and istockphoto lack?

Your product quality and development budget will suffer if you can't scale. There are clipart sites that let you buy 30 credits for $30, with many items sold at 1 credit.

Some people only want something cheap. They can get free at unsplash.com. Some people are willing to pay a little, some are willing to pay more, and some are willing to pay a lot but want exclusivity (no-one else can use the image). Ideally, you would charge people based on the max they are willing to pay. This is why I said usage based pricing - those that show/use the image a lot pay more. But this is hard technically to implement.

So you have to pick your target market. It's possible to have tiered pricing. Decide different features for each tier (it could be resolution, modification rights, licensing terms - but really you don't want to spend money taking people to court).

Do a survey of your competitors.

Pricing is a hard problem to solve - auction, reverse-auction, micro-bidding sites try to tackle the problem of maximizing the sale price.

Say your goal is $100K. Then is it easier to sell 1000 items at $100, 10,000 items at $10, or 100,000 items at $1?

I think there have been discussions here about this. They said low pricing is a trap. You spend more on marketing because you are targeting a wider audience. Cheap customers require more support and are less loyal, etc.

I'm being very general - read more on IH and you will see.

Some comments on price and revenue here: https://www.indiehackers.com/forum/im-tired-of-reading-posts-that-talk-about-rocketship-revenues-in-too-less-time-or-is-it-just-me-b24dd171b4?commentId=-LJoDI6B2q48ZHG2qyBS