Space and data are at a premium on every smartphone out there. It may seem counter-intuitive that you may want to lower image quality in iMessage, but think about it this way, when you’re just adding for fun photos to chats, do you really expect the person on the other end to save and print them out every time? Probably not. Some photos, maybe there’s an incentive, but if it’s just a picture of some flowers or a cup of coffee or a landscape shot or a look at my silly pet type photo, why would they need to?
If they’re going to want to save anything, it’s probably going to be pictures of people, that’s just human nature. Front facing cameras tend to have smaller MP cameras anyway. So why not agree to use lower quality images in iMessage and save yourself some bandwidth and local storage. Now keep in mind that this solution only affects messages sent from one iPhone to another, and it’s an all or nothing scenario.
Turn it off and it’s going to be off until you remember to turn it back on. You cannot, decide that you’d rather have it apply to only certain contacts for instance. To set it up just open up Settings and scroll down until you see Messages and then tap to the next screen. From there, just scroll to the bottom of the screen and find the section marked Low Quality Image and then tap to turn it on. That’s it, it’s not a super noticeable difference on a small screen, the only place where it’s going to become noticeable, is if you’re trying to back it up somewhere.
Sources: How to Geek