My view is that most images are decorational in the terms that we're talking about. You can view a photo of a burst dam, to take one of my examples, but a description of the scene should be in the article already, and therefore any further description is superfluous. The "should" in that sentence is doing a lot of work though - I don't think people consider this kind of stuff much at all, not on a deep level about why they're doing things. My first proper web job was in the BBC in the late 90s, and it was a really steep learning curve but has stood me in really good stead when considering this kind of stuff.