Before Instagram I had hundreds of photos on my computer. Yet, Instagram wants you to post photos from your cell phone. BUT, I found a way to override this. Let me show you how. You are going to LOVE this! Get Step-by-Step below.
Go to Google Chrome
Click on View (at top of your menu)
Click on Developer to Developer Tools
You will see to the right a Toogle Bar. Click the Mobile photo.
You are now in Mobile mode.
Go to Instagram.com
From here you can add photos from your computer to your Instagram.
REMEMBER to switch back after you have done this, or your computer will stay in Mobile mode.
Let me know how it worked!