My wife wishes I had waited for my son Clay to stop scowling and shivering before I took this photo of him. To her it's a "bad" picture. The fact that it's sharp and well-exposed is irrelevant. I think it's funny, and because it defies expectations of what this sort of picture should look like, it's more interesting to people outside my family than it would be if he was grinning. What do you think?
How do you know when you've taken a "good" picture? All that some photographers require is that the image be in focus and correctly exposed. This used to be more of a challenge in the days of manual focus lenses and manual exposure. Now that auto-focus and auto-exposure are the norm, even a complete neophyte can take a good picture by this definition.
That's a good thing. But let's say you aspire to something more than technical competence alone. In that case you might define "good" as above-average. You might look for an image that was well-composed, that had an unusually attractive subject, or that was well-lit. Professional photographers might refine this definition to include photographs that meet or exceed client expectations or that serve the designated purpose. Professional or not, the intent is usually to please the viewer.
Then again, you might belong to the coterie of photographers (and photography critics) that evaluate a photograph based not on whether someone likes it or not, but rather by how well it qualifies as "art." The word art in and of itself puts us on a slippery slope, so let's just say they're looking for photographs that challenge assumptions, that force or invite us to see things in new ways, and that leave us amazed and fascinated by what others can imagine and create.
So I will conclude with where I began: How do you know when you've take a good picture?
While you're pondering the answer, here's a hint: It's a trick question--yet it's still worth answering, if only because it might help you clarify how you judge the quality of a photograph, whether it's yours or someone else's.