Thanks!You go to the drawings, painting, pixel art section here, look for a thread that is taking requests and ask for custom art or you can use a public use one from another member.
These are the most frequent I've seen:
or I've seen animated slimes, I'd ask for permission to use someone else signature though.
You can add it to your signature by clicking your profile icon on the top right and going to the signature tab, adding the image, and saving it:
If you are talking about these:I've seen animated slimes