With what understanding I have:
Image is an artifact and used in a generic sense, referring to a representation of any real world object.
Images may be two-dimensional, such as a photograph, screen display, and as well as a three-dimensional, such as a statue or hologram. They may be captured by optical devices—such as cameras, mirrors, lenses, telescopes, microscopes, etc. and natural objects and phenomena, such as the human eye or water surfaces.
The word image is also used in the broader sense of any two-dimensional figure such as a map, a graph, a pie chart, or an abstract painting. In this wider sense, images can also be rendered manually, such as by drawing, painting, carving, rendered automatically by printing or computer graphics technology, or developed by a combination of methods
Raster, on the other hand, is a data structure used by computers to create/manipulate images in digital form. They have a rectangular grid of pixels, representing different values.
So what we call a picture or image in real world, comes to be known as raster in digital parlance.
That's the reason they are used interchangeably...!
You can refer the Wikipedia pages for image and raster to get more insight.