The developer states it's the first and only app in the Mac App Store that lets you upload photos to Instagram from your Mac. It's certainly the first third-party app I've encountered that does such a thing. Other Mac apps, like Instastack and PhotoDesk let you view your Instagram feed and comment and like photos, but none let you post photos because Instagram prevents third-party apps from uploading to its API.
Enterprising developer Caleb Benn of Anobot LLC found a way however, and has released Uploader for Instagram. The app costs $4.99, £3.99, AU$6.49 in the Mac App Store.
Uploader for Instagram installs an icon in your Mac's menu bar and adds an item to the right-click menu. To upload a photo, right click on the file and choose Share to Instagram or Services > Share to Instagram.
The app then opens a small window where you can zoom in to crop your photo. By default, the app crops landscape-oriented photos to fit the Instagram square, but you can zoom out and post the full, rectangular image.
You can click the camera shutter button to the right of the zoom slider to select one of 47 different effects for your photo, but they are more akin to wacky Photo Booth effects than Instagram filters. You can also also click the Camera button to the left to snap a shot using your Mac's iSight camera to upload.
When you have your photo ready, click Done. A new window will then drop down from the app's menu bar icon, letting you add a caption. To upload it to Instagram, click Share. You get a notification letting you know your upload was a success.
Now, you have an easy way to share with your Instagram followers the photos you took not with your dSLR, not just with your phone.