I don't have my Laptop with me (on a phone), but this might help..

https://steemit.com/steemit/@fiveboringgames/how-to-embed-a-pic-or-a-gif-in-a-steemit-post

Seems like you might need to write .gif on the end. If that doesn't work, let me know and I will get back to you tomorrow

Also, if you want to search for something, I literally went to Google and typed Steemit GIF..