One way to frame it is: "how would I describe this image to somebody sat next to me?"

The question to ask is, what a sighted person learns after looking at the image? The answer is the alt text. E.g if the image is a floppy, maybe you communicate that this is the save button. If it shows a cat sleeping on the windowsill, the alt text is yep: "my cat looking cute while sleeping on the windowsill".