Welcome to Tech-Forums.
Signatures are disabled for new members. This was put in place due to spammers. Once a member reaches 40 posts, signatures become available, as do private messages.
If you are wanting to add an avatar, you will have to upload a GIF. As far as I have been able to tell, no other file type will work, and URLs don't work at all. This will soon be rectified, but we have to live with it for now.