Light, angle, resolution, and the difference between a snapshot and a usable source.
Most family photos are made for memory, not for painting. A reference photograph an oil portrait can be built from is a different object.
Almost every commission begins with the same question: do the photographs you already have work, or does a new sitting need to be arranged? The honest answer depends less on the camera and more on four conditions, light, angle, resolution, and intention.
A portrait painter reads a face by reading the light falling across it. Direct overhead light flattens the structure of the brow and cheekbones. Phone-camera flash erases the shadow side of the face entirely. The reference photographs that translate well into oil are made under soft, directional light, a north-facing window, an open doorway, late-afternoon daylight on a porch, where one side of the face is in clear illumination and the other holds gentle, readable shadow.
The camera must sit roughly at the subject's eye level. A reference photographed from below makes the sitter look imperious; from above, diminished. Neither reads as the person their family knows. For seated portraits the painter usually wants the lens slightly above the eye line, mirroring the angle from which the finished painting will be viewed once it is hung.
A reference photograph is not the painting. It is the instrument the painter uses to see the sitter accurately.
An oil portrait at 30 × 40 inches asks the painter to render eyelashes, the precise color of the iris, and the soft transitions in the skin around the mouth. Reference images for a painting of that scale should be high-resolution originals, not screenshots, not social-media downloads, not stills pulled from video. A single sharp file from a modern phone, held still, in good light, is usually enough. A blurry image cannot be sharpened by painting it.
For living sitters within reasonable distance of the studio, an in-person session, or an artist-directed photography session, almost always produces a stronger painting. For memorial commissions, where the sitter has passed, the studio works with whatever family archive exists, gathering several references and triangulating likeness across them rather than relying on a single image. In both cases the goal is the same: enough accurate information that the painter is making decisions about the person, not guessing about the photograph.