Image Quality of Images Shared via Messages MMS
Typically the size of the SMS package is controlled by the network provider but it is on average 3 Meg per message.

So a single photo will look sharp. How ever if you try to send multiple photos they have to fit in the 3 Meg bucket, and will be reduced in size.

You best option is to use Dropbox or Google drive, Photobucket ect.... if you want to send larger then 3 Meg files.

Basically select all photos and just upload to one of the services and then share (send link) to the other party.