As stated there, there are two options, either of which is easier than what you're doing. First, you can use the "Add image to post" feature, which works through imdbb. Second is to use(as you were trying to do)the "Upload file" button, which works through Cloudinary. However, you must first set up an account with Cloudinary, then link to that account with your profile here. Detailed instructions to do this are in the link above.