It is generally not advisable to create a painting from a photo without obtaining permission from the photographer, as they usually hold the copyright to the photo. In terms of US copyright law, only the owner of copyright in a work has the right to prepare or to authorize derivative works based on the original work. Therefore, creating a painting based on a photo without permission would likely infringe the photographer's copyright. It is not clear how much of the photo needs to be changed in the painting to avoid copyright infringement, as the copyright law does not specify a minimum amount of changes that must be made. It is recommended to obtain permission from the photographer or to use your own original photos as a reference for your painting.