Learn English – What’s the difference between “to frighten” and “to scare”

The OED defines them as (note these are only partial entries, as both words have several meanings. However these seemed to be the ones more appropriate for this discussion):

picture: An individual painting, drawing, or other representation on a surface, of an object or objects; esp. such a representation as a work of art. (Now the prevailing sense.)

image: An optical appearance or counterpart of an object, such as is produced by rays of light either reflected as from a mirror, refracted as through a lens, or falling on a surface after passing through a small aperture.    Such an appearance may also be a mere subjective impression on the sense of sight, as an after-image (q.v.), and the negative image or accidental image seen after looking intently at a bright-coloured object, and having a colour complementary to that of the object. An image produced by reflexion or refraction is called in Optics a real image when the rays from each point of the object actually meet at a point, a virtual image when they diverge as if from a point beyond the reflecting or refracting body.

So I guess image refers more to the optical process, while picture is the actual painting/printed photograph etc.

Of course image is also used to refer to image files in informatics.

Why stop with two? You can also speak of the prime reason, the central reason, the primary reason, the principle reason, the foremost reason, the predominant reason and the pre-eminent reason. Like main and chief, they have much the same meaning, but are likely to be found in different contexts. Only a corpus search can reveal what those contexts might be.