When you go to send an MMS message, and you attach a picture to it, it resizes the picture IF it is larger than the size you have specified in settings. Like if you set the maximum file size for MMS messages to be 1.2mb (the largest you can select), and the picture you are trying to send is larger (say, 2.3mb for a typical 8mp high quality photo), it will drastically resize to something like 20kb.
To circumvent this, you can do one of two things.
After taking a picture, crop it. You can crop it the full size of the picture, and it will compress the image down to about 700kb-1mb or so. When sending THIS image, it will not be resized.
The other option is to set your camera to something like 5mp and fine (instead of 8 and high). This is the most convenient way to do it, as long as you mostly use your camera for MMSing. When you occasionally want to capture a "keeper", just remember to dial it back up to 8mp and high.