I do not think photos ever become paintings. But...photos can be photo-illustrations. And of course at times, photos can have painterly qualities or characteristics. Painting uses paints or pigments on a medium,like canvas,or wood,or concrete, or something else. Photographs are made with a camera and lens of some type, and are captured on "some type" of medium: film,plate,photographic paper,or an imaging sensor of CCD or CMOS type, and there's an intermediate stage, and a final,printed stage. Painting has no intermediate stage between imagination/conception, and the final stage...a painting is painted...there exists NO "negative", no "RAW file", no "SOOC JPEG" file. Mixed media pieces on the other hand, differ from both photos and paintings, and can easily share the characteristics of both photos, and of paintings, or drawings, or sculptures. Mixed media was all the rage at one time, and it never has completely disappeared. It's still out there! In another sense, some people like to differentiate between the practice of photography, and the practice of digital imaging. While similar, photography is subtly different from digital imaging. Perhaps the OP's question is really more about defining the area where photography crosses into digital imaging? Where "Straight" work crosses into "manipulated" work?