If the alt text is added for accessibility purposes, such as for users using a screen reader, I would suggest the below practice. It's the house rule of one of my client when working as technical writer-editor, but I find applicable to other project as well.
For decorative images that no meaning content wise, don't add alt text. (people on screen reader don't need to read it)
Add alt text for those images that are important in context, so people depending on screen reader can still know what it is about.
If there are consecutive images that uses an identical alt text, only add alt text to the first image and omit the alt text in the consecutive images. e.g., if you have 3 images of a slightly different angle of a vase showing consecutively, write "picture of a vase" on the first image and omit alt text in the second and third image, so that people on screen reader don't have to read it three times. (just one time is enough
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To put it simple, try disable the CSS and see if your content make sense! If it does, it should work great for a screen reader too.
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