Hi,

By default there is option to add the signature of the customer from Online. In the Other Information tab in the sales form, you can see a Boolean field named Online Signature, once you enable it, the customer can sign the quotation online . Once you click the Preview button you can see it.

If you are looking an option where you can sign, you can change this or refer this custom module in v11: Sale Order Digitized Signature

If you just need an option to add an image, you can add a binary field in the sale.order, this can be done from code or from UI, for adding a new field from user interface see this: How To Add Custom Field From User Interface in Odoo

Thanks