Unfortunately, no, you cannot disable the ribbon. While the ribbon hides the bottom of the image, it's purpose is to allow you to scroll through images/video that you've 1) received in that conversation and might want to 2)save. Images automatically do NOT save to Gallery. That would probably make your Gallery explode and max out your space. Nobody wants that!
IF you tap on the image tap on the larger image a couple times, the ribbon goes away and you can now flip through the full size images.
I hope this helps.