If you are using the normal profile edit boxes (like "About Me" or a custom bio section):
Get the image address: Right-click your blinkie and select Copy Image Address (or upload your own to your FriendRewind gallery/ImgBB first to get a direct .gif link).
Go to your Edit Profile section on friend rewind
Click inside the small text box where you want it to go.
On the text formatting toolbar at the top of the box, click the Insert Image button (it looks like a tiny picture icon).