Someone made the comment that it didn’t matter and pointed out—disdainfully—that some curators also waxed indignant when people refer to a painting as a picture. As long as the communication is clear, what’s the issue, the person asked.
Clarity and precision.
A picture can be a painting, a photograph, a drawing, a petroglyph, etc. You get the picture. A painting is a picture with certain attributes, starting with how it was made.
The person asking the question fails to recognize general and specific terms. Let’s hope the FB discussion showed her the light.