I'm assuming you are talking about signatures.
They can be added from both the old and new website.
Here is a link to the new website's account settings, you can edit it under the signature section:
https://osu.ppy.sh/home/account/edit
Here is link for the old website (the preview button is broken for images):
https://osu.ppy.sh/forum/ucp.php?i=profile&mode=signature
You can use BBCode in it to include a picture, just like you would if you were posting an image in a regular forum post.
[img]img-url.png[/img]
Sidenote: Signatures are disabled on certain subforums, such as this one.