Instagram wasn't designed to post from the Mac desktop to Instagram. However, there is a way to get it done without too much effort.
How to Post on Instagram from Mac
- Go to Instagram.com using the Safari browser and Login to your account
- Open Safari and click Safari (upper left corner of screen) > Preferences > Advanced
- Check "Show Develop Menu in bar"
- Close this and go to: Develop (in the top nav) > User Agent > Safari - IOS 10 - iPhone
- The Instagram site will reload or manually refresh the site.
- Click the camera icon on the bottom and a window should open asking you to select the picture you would like to upload to Instagram.
- Review the preview of the image, edit, add a caption and then click Share.