According to the
Community Guidelines...
Signature, Avatar, Profile, Username and Account Standards
Community Standards apply to all sections and features of the website including avatar and profile images, signatures, usernames and other account features.
1. Signatures must be setup in your profile rather than manually added to your messages. They will appear at the bottom of every post you make.
2. Signatures should be kept to a maximum of three lines (including blank lines) and contain no more than one links.
No images allowed in signatures.