All you have to do is sign in using the username and password you use to access your profile and then select the photo you want to upload. You can’t just upload anything: it has to be a square picture with a minimum of 640 x 640 pixels, thus abiding by Instagram's regular upload parameters.
On its interface, Gramblr itself recommends a website that squares your image, although you can also do it with any photo-editing tool. Once your picture is ready in JPG format, simply press the upload button.
After that all you have to is add the caption, and once it's uploaded you'll have the same options as on the mobile version: share the picture on Facebook or Twitter.
There are no options for effects so you'll have to settle for a filter-free result, yet it's still, for now, a good option to upload pictures from your PC, as the Instagram web version doesn't allow this feature.