Use detail where it earns its place
A portrait or animal photograph may need smaller cells to retain an eye, nose, or distinctive marking. More cells also mean more labels and a busier sheet. Inspect those essential features first, then decide whether the extra complexity helps enough to justify a more demanding painting reference.
Keep the palette practical for real paint
The generator samples image colors rather than selecting paint mixtures. Sixteen groups may preserve more shading, but they do not automatically correspond to sixteen useful tubes of paint. There is no separate numbered color legend. Keep the original photograph nearby and prepare your own small set of mixtures before tackling the full study.
Remember the output ceiling
The paint reference is limited to 960 pixels on its longest edge and saved as PNG. It has square cells, not vector object regions. Very large prints will not gain detail by enlarging this raster file.