Do I need to use Instagram?

Just FYI, static images can actually be posted to Instagram from your computer's browser using Chrome. Only JPG files will be recognized though. Go to your profile on Instagram in your browser, then CTRL+SHIFT+I to open the Inspect panel. Then use CTRL+SHIFT+M to toggle the device toolbar to convert the page into Tablet/Phone mode. Then refresh the page. You will see the familiar touch screen controls appear at the bottom of the screen. Use your mouse to click the plus button, and it will open File Explorer on your drive. From there, you can pick an image to upload right from your browser.

There are ways to get Instagram working as an app on Windows 10 also for uploading video, but I don't have time right now to explain all that.

I find it hard to believe you'll get many (any) sales from social media. Buyers are mostly 3D users, with a specific need, in a hurry - they don't browse social media and buy on impulse.

If you produce the regular type of stock (already overly abundant) then social media probably has no extra value to get more noticed. With “regular stock”, I mean the real world items like cars, furniture, household products, etc.

However, if you make more artistic types of things less known to be available as stock media, then it makes sense to spread the news. In this case using Pinterest can bring good results for spreading the links to your items. Note however the stuff needs to be visually appealing to some degree in order to get pined allot, probably same on Instagram. The regular items usually are not that appealing and don't generate that much interactions (likes, shares, followers) so probably pointless to use those for advertising. If things are visually appealing to some degree then it can also do good on Art portals like ArtStation, etc.

I imagine if you spend the time (I never did) to understand how to “successfully” use social media for advertising, then it probably brings better results. My personal take on that is, first make extensive enough offerings, then make own store, then learn “successfully” advertising on social media and promote own store.

In meanwhile just post on Pinterest, art portals and in some groups on Twitter, Facebook, etc.

Know your target audience, as an author I have spent many hours in boring webinars learning how to publicise my books using social media and other similar methods. Lots of people think they can post a product they have made whether it be a 3D model or a cats waistcoat on their personal Facebook, Twitter, Pinterest etc. and it will sell by the tonne. Not true. If used badly, social media is as much use as a chocolate teapot.

1. Know your product
2. Know your customers
3. Know where your customers go.

Example; If you make game-ready models, you know that the target is game-devs/ advertisers (and a few others). Your Auntie Joan might like your model on Facebook but she sure as hell isn't going to buy it! Join game-dev groups and post your model there CAREFULLY. Don't get kicked out of the group for spamming.
Join Reddit groups that are concerned with your "theme" of the model(s) and answer a few posts before wading in with "Look at this I made, now buy it".

A blog is also a pretty good way to sell your stuff and also a Youtube channel. My blog gets around 1200 or so hits a week because I don't spam my models, I do spam my blog because I can get away with a link in every post more than I can a massive screenshot.

I don't post a lot on social media but when I do, I tag appropriate bots that will repost my links to a lot more people that follow me.

Good luck.