Well, Instagram has recently launched a dedicated app for Windows. However, the app is available only for Windows 10. You can access your Windows app store and install the app on your Windows 10 PC. However, it misses the feature you are searching for. There is no option for uploading a photo from the Windows application. It seems as if Instagram focusses mainly on the mobile apps for the upload option.
However, there are a couple of alternate methods you can use if you want to upload your photos from the PC.
Run Instagram Android App on your Windows
Yes, you can run the Android apps on your Windows PC. You can Android emulators for the purpose. Bluestacks is one of the popular Android emulators. Install Bluestacks (or any of your favourite Android emulator) and install the apps of your choice including Instagram.
This is a popular third party app for Windows. Once you download and install the application, register using your email ID. Then, link your Instagram account to the application. That does it. You can now use the app to upload pics to your Instagram account from your PC.
This is yet another app that does the work for you. Please note that the Gramblr is not authorised by Instagram. It may record your information. I would advise you to proceed with care. However, the app is one of your best mates to upload photos to your Instagram account on your PC.
Live....and Let Live!