Instagram, the popular photo sharing service, doesn’t allow to upload photos from web client.
But you can do it using a free software for Windows and Mac, Gramblr
Once installed you just need to login using your Instagram username and password (pic1), then choose the file to upload (pic2): the photo must be in JPEG format and less 500kb, it must be 650px by 650px (it’s suggested a free web service to do it but you can use any graphic software that does resize/crop).
Then add a caption (pic3) and lastly you can share the pic in social networks as link or HTML code (pic4).
As you can see the photo will be shown in your Instagram profile (pic5).
To be 100% as Instagram the program lacks the feature of apply the vintage/retro effects to photos but you can first process your photos, for example using Instant Hipster a nice set of Photoshop actions (10 free actions, the full package includes 19) and load them in Photoshop.