I recommend that you consider using a method other than PhotoStream to sync photos among IOS and macOS devices. Photostream, when used in a particular way to transfer photo files among IOS and macOS devices in multiple steps, can corrupt the photo files.
A few months ago I posted my experience with corrupted image files due to PhotoStream (but not iCloud Photos, formerly known as iCloud Photo Library). See detailed explanation here: https://talk.macpowerusers.com/t/another-cautionary-tale-about-icloud-photos-photostream/14361?u=arthur
The problem I experienced with Photostream occurred in 2017. I do not know if the problem has been resolved since then. Even if it has, it is possible that some degree of image file compression might be applied under certain conditions - slow wi-fi connection, for example. You should always be aware of image file integrity, since it is a common practice to compress image files. A good example is sharing files via email - the Apple Mail app compresses image files by default. I would not trust Photostream to guarantee image file integrity.
Edited to add: iCloud Photos (formerly iCloud Photo Library) is a different issue. It is my understanding that iCloud Photos maintains the full-size image files in iCloud, even though compressed images might be stored in IOS or macOS devices. I do not know of anything PhotoStream by itself does to maintain file integrity.