Is it permissible to request a painting from someone I know, such as an artwork of Imam Ali raising his hand during Ghadir Khum, or would that be deemed sinful?
Such paintings are never real pictures of the Imam (AS) but just imaginations of the painters. In some cultures like Turkey, Iran, Iraq etc they use such paintings to remember the Imam, while in many other Muslim cultures like in India, Pakistan, Bangladesh etc, they don't use it. I personally don't use such paintings for many reasons including not trusting the details of the paintings and to avoid misunderstandings among Muslims from different cultural backgrounds. Those who use such paintings are not doing a sinful act but trying to remember the Imam (AS) in their own cultural eye.
Wassalam.