How to post your Instagram photos from a computer is very convenient. I will show you step by step an easy way to do that if you are on a Mac
For Google Chome (posted by Jonathan Yap) :
Thank you for taking the time to make this video. I've been using a similar technique on Google Chrome from my Mac. Just open Instagram on a tab, then click on the 3-dot menu, go to "More Tools", and click on Developer Tools (Keyboard shortcut: Option + Cmd + I) The developer toolbar will appear on the right of the screen. Then make sure to click on the icon that looks like a tablet & smartphone together (Keyboard shortcut: Cmd + Shift + M) to toggle smartphone view. The upload process is pretty much similar from here onwards. Hope this helps.