First of all you need an animated picture you can search for one e.g. on https://giphy.com or upload your own into a cloud. you can use https://imageking.eu (no registration required) (If you upload your picture to another hoster make sure that it is publicly visible).
Choose a gif and click on "copy" link. Now copy the gif link. if you use imageking copy the link from the field where "URL" stands.
Now open the account settings on steemit and paste the url at "Profile picture URL" and click on Update
