You're right. Trying to use "insert image" option to insert Google Drive links will not work. There are 2 things you could do to work your way around this. first is to use "insert media" option and paste your google drive link there (I don't recommend it though because google drive links will create a humongous space around your gifs in the signature area), the second option is preferable and that is what a few of the members did on this thread, upload your gifs as attachments in a post when replying to this thread and then use their links in the "insert image" option in the signature area. I'm going to do the same. hope this helps.