Step one:Find the picture you like, Like I seen funny pictures with captions too be a hot topic.

Step two: Copy the link of the pic or gif

Step three: Go to planet minecraft and click on "My Account", then click on "Edit avatar, profile & settings."

Step four: Click on the Tree pic and a box will pop up. Paste the PIc or Gif link in the "Image URL" box

HOPE THIS HELPED!!