To add a signature to your account you must go to Settings > Manage Accounts. Select the desired e-mail and tap Signature.

Aqua Mail supports plain text, rich text, and HTLM input in the Signature module. Photos and links can also be added. For all non-plain text signatures, text formatting must be enabled beforehand.

Note: For HTML, make sure the code contains the <html></html> tags in order to be recognized. If correct, you should see a preview of how your HTML signature will look.