hardyian

There are two primary parts to Google SEO web design strategies: on-page optimization and off-page optimization. SEO techniques can be divided into categories: white hat techniques, those that are acceptable by the search engines and are the recommended ways of improving rankings, and black hat techniques, those which are deceptive and result in Google penalties or being banned by the search engines. When SEO is done correctly and built into the web design process, SEO services will add value to the website experience by making the website search-engine friendly, a better user experience,  and greatly helping businesses achieve their online marketing goals.

Loads of things.
Start with this: https://developers.google.com/search/docs/beginner/seo-starter-guide

You really need to do more research. Google has been telling people for a long time that guest blog posting is ineffective.

You basically need a platform that allows marketplace selling and payment distribution, neither Shopify or Squarespace offer that currently. The biggest question is how you're going to get both types of users required to make this work? This is the first step to work out, if you build something, there's no guarantee of getting even a single user without the right marketing strategy.
I would be looking to build a minimum viable product (https://www.productplan.com/glossary/minimum-viable-product/) to test your idea before committing too much. Speak to artists about selling their artwork through a simple store, with an agreed commission. This should give you an indication if you're heading in the right area, before having to invest in it heavily. The best way to validate your idea is through feedback as early as possible, you'll likely find things that need to be changed, or assumptions about your idea that didn't turn out to be correct and you'll be in a better position to adapt.

Yeah the infrastructure required to handle mass uploading of image files will be substantial, especially if you expect it to scale upto a large number of users.

I suggest you pair up with an engineer who has a good understanding of a platform such as AWS.  I hope you have deep pockets as to get such a system off the ground in a way that can scale won't come cheap.

I definitely won't go for any locally hosted server, you want a cloud based solution, like AWS as mentioned already, essentially you'd want images stored in an S3 bucket and a database (Dynamo) that holds associated information to the entries etc.