What signature or symbol did Leonardo da Vinci use on his paintings?

Answer 1

No painting exits that contains his signature. As it turns-out, Leo did "sign" his work, but not with his name. We should note that several of his drawings were "signed" by other hands, later-- in order to make them more valuable.

Most artists were generally considered in no higher regard than other craftsmen.

The artist was announced by the benefactor who purchased the work, and was generally recognized by his style and reputation alone.

Almost all work at the time was anonymous (without being signed), and usually performed for the church or the rulers of the land. Art done for rich merchants, for instance, would be given in their name to the church-- they were called 'donors' and often were pictured in the works-- off to the side and smaller than the religious subject. Private commissions and portraits were rare.

Answer 2

Yep. Not always, but he did often.

Answer 3

Usually not