Your signature can be stored in your user profile to be stamped when using forms, quotes, invoices and purchases.
You can upload your signature through the Web Portal by going to Admin > All Users > select the edit arrow for the user that you would like to add the signature to > scroll down to Signature and re-sign or upload an image > Save.
When using quotes, invoices and purchases through the CRM or completing a form via the user profile that you are logged in as, you can then sign your signature with the saved signature stamp.