I have the Galaxy S8+ and my wife has the LG G6 on T-Mobile and we have the same issue. It is really frustrating and has to be something on Google's side.
Yeah it's frustrating, see if Samsung and LG offer there stock text messenger in the app store. HTC started offering the stock HTC messenger in the app store and I downloaded it on my HTC 10, but it wasn't in the app store on my HTC U11. If the messengers are not available see if any of your friends or family has a Samsung or LG that has the base messenger. If they do, have them download App back up and whatsapp and you will have to download those as well. Have them open up app back up, select the messenger app and send you the apk file through whatsapp so you can download on your Samsung and or LG. That's it, I did this with my HTC 10 to download on my HTC U11
Sorry to chime in a bit late on this matter, many of the devices we see are being shipped with Android Messages as the default Messaging client because RCS (Rich communications Services). RCS will utilize this application once launched, enabling much more than just pictures and text to be shared. I'm not certain on how things will change as for the compression factor. I can say in my personal experience if I have a picture that just needs to be full resolution I will send it via Email.
Thank you very much for your information, but sending a full resolution picture through email defeats the purpose of send quality pictures through text. If sending quality pictures through text was not possible, I would understand, but I have been sending quality pictures through the base text messenger app, for instance; I have the HTC U11 and now using the base messenger app from HTC and my pictures are great and the time sent is quick without lag. I think Google just need to test their messenger app on certain brand of phones to make sure it's working properly.