At the bottom of the posting UI there's a little arrow that opens "Additional Options." Click on "choose file" and select the picture you want to upload from your computer. Once it's in there, it won't show in the preview, but it'll show once you post it.
There's also a way to do this without the attachment, but it has to be a picture that's already been posted before, like a reaction image. Just go on google images, find the image you wanna post, click on "full sized image", and copypaste the URL from that into the [ img ] and [ /img ] tags.
