It's basically what Craziness_all_the_way said in the comment above. Copy the embed codes of the meme/gif and paste it into a comment. Then delete the parts you don't need in the front and back, and post the comment. Once you refresh the page it should turn blue and be a Click to show.
But here's a meme with some more detailed instructions in the comments:
imgflip.com/i/vk8n9
If you're still having trouble with it after you read those instructions, ask me again about it. :)