A good image should grab you.
If you have to read a book, or attend a class, or visit an art gallery in order to appreciate an image, then it has failed as an image.
Simple as that.
You may need to do all of the above in order to understand some images (symbolism, context, references, etc.) but that’s a different issue because, though you may not understand an image, you may still appreciate it as a form of expression.
Photography is a visual art after all.
The first — and most important — order of appreciation for any image (photo, painting, etc.) is therefore at the visual level. If it doesn’t pass that test, it’s literally not worth looking at.