How do I upload a photo from my Photos app to Instagram?

You need to "trick" Instagram into thinking you're using an iPhone. That's done with something called a "user agent".

Open Safari.

Go to Safari -> Preferences -> check "Show Develop menu in menu bar".

Go to Instagram.com

Go to Safari -> Develop -> User Agent -> Safari iPhone.

Your webpage will reformat to a large version of what the iPhone screen would look like. It is not permanent, and is only for the open Safari page.

Click the '+' sign to add a photo.

Note: You can't add the photo directly from your Photos app. You'll have to drag the photo somewhere else (like your Mac Desktop) and attach it to Instagram from there.

Hi

The desktop web page doesn't allow uploads. Instagram is intended primarily for use on a phone.

However, see the article here for some workarounds:

https://www.macworld.co.uk/how-to/instagram-for-mac-3641569/