You have to have the image made already, and then you click Show embed codes, then copy and paste the BBCode(forums), and delete everything before and including the [img] so that it starts with http:, and also delete everything after and including the [img] so that it ends with .jpg
anona247 explained it in the first comment under one of her images. You can check out the full list of instructions, here:
imgflip.com/i/vk8n9