You have to post something in the GN text box first, then use the choose file command below the text box, then preview. If you are happy, then post. It's very easy to do.
There is a limit on pixel size, so I have had an error message in the past and photos wouldn't post.
Click on the paperclip icon in GN (next to place where the smiley things are) that should enable you to go to your photos and choose the one you want to upload.