A picture is not reserved for something we can see with bare eyes - an infrared camera produces a photo, we don't call that an illustration - such cameras are used in military, geology, earth mapping and agriculture, etc. Most images in industry and astronomy could never be seen with bare eyes, because our eyes can see only 1% of what our instruments can.

Ofcourse each wavelength of light works a bit differently, like with the X-ray example, you need to understand how X-ray works to understand what you are looking at.

A model or illustration is something that would be product of our calculation or imagination, that was given visual form.