I generally tell people that alt text is one of the few places where accessibility can be as much art as it is science. As other commenters noted, context is important. Images will most likely be consumed with its surrounding context. If the image is bringing nothing new, consider marking it as decorative.
But if it isn't, express the intent of the image along with its content. Let's say you're back in the office, and describing a photo to someone sitting at a desk across from you. How would you describe it? It the point of the image that it's art? Is it the structure and layout of the scene? Is the point just to identify what a chrysanthemum looks like?
I'm often reminded by a colleague that if you get too wordsy with alt text, or it doesn't seem important or valid, a screen reader user can easily skip past the image and move on.
At the end of it all, the two most important bits seem to be: making any effort is better than not trying at all, do your best and you'll get better at it over time; and remember that even though a screen reader (or some other mechanical assistive technology) will parse the text you write, you're doing this for the person on the other side — they love to laugh, smile, have fun, learn, and understand things just like you do. :)