Click on the third icon from the right, the one with the tree in it. When the page opens, click either "from computer" or "from URL". You seem to want from computer. Click "choose file" and then click or double click on the picture you have stored. Click "upload file" and the click "Post reply"
I guess that is my issue. It only has the add an image URL option, not from computer. I have no option to browse or choose a file, only to type a URL into the box. I was initially trying to post from my phone, but I also tried my laptop as well. Same issue on both devices.