There are two considerations when draw celebs. The copyright of any reference you may use, and the right of publicity of the celeb.
If one chooses to use protected material/references, the same typical fair use considerations should be made. Typically a good artist can transform reference material into a new creation (transformative test
). An easy example of transforming a photo of a celeb into something new would be when caricaturists et their hands on it. Another less obvious example of transforming photos into new original artwork would be what Andy Warhol did.
The second consideration is the right of publicity. There are both federal and state "right of publicity laws", so depending on where one is the details could be different. But this law essentially states you cannot use a celeb's image or likeness for commercial purposes without consent. So essentially you can’t make it seem like an actor is endorsing a product or put in some ad without permission. There are exceptions to this law like has already been mentioned.
Typically the exceptions read like this: "a use of a name, voice, signature, photograph, or likeness in connection with any news, public affairs, or sports broadcast or account, or any political campaign, shall not constitute a use for which consent is required under subdivision."
So essentially it's like fair use. On top of that first amendment rights always trump the right of publicity (So a cartoonist can lampoon an actor and recieve 1st amendment protection). Also in many states like California, they write in specific exceptions for artists to be able to paint a likeness in "single works of art". This is California’s written exceptions to the right of publicity:
(n) This section shall not apply to the use of a deceased personality's name, voice, signature, photograph, or likeness, in any of the following instances:
(1) A play, book, magazine, newspaper, musical composition, film, radio or television program, other than an advertisement or commercial announcement not exempt under paragraph (4).
(2) Material that is of political or newsworthy value.
(3) Single and original works of fine art.
(4) An advertisement or commercial announcement for a use permitted by paragraph (1), (2), or (3).
Some states that include similar exceptions for artists are Illinois, Nevada, Washington, Indiana, so these places its absolutely (Without question or grey area) legal to paint a celeb. (Prints however is in the grey area) Law experts believe a limited number of prints would not be any different than a single work of fine art" but there are no cases proving this.