Conforming the proportions of a single tile of a texture map to those of a surface

From what I can tell, a picture (say an image on the face of a stretched canvas) is most efficiently made by creating an image on a plane without thickness, which is then positioned on the face of an object representing the canvas.

I thought I’d be able to apply an image to a box and have it automatically adjust its proportions to those of the plane of the box to which the projection is perpendicular, but it appears not. I couldn’t get the texture mapping widget to launch either.

It seems a texture will automatically stretch its proportions to fill a rectangular surface if and only if that surface has been formed from a planar srf command, or if the texture has been loaded with the picture command.

I’m not sure I have this correct, but it took a lot of experimentation to figure out the above. The help docs and tutorials don’t seem to quite cover it, but if there’s a place where this is thoroughly explained, I’d love to know.