Remember, @lanodan, that an exclusive rights holder (e.g., a copyright holder) always has the permission to issue any material under any license (or many different licenses) or not.
This is what allows “proprietary relicensing” to happen at all.
That sad, the other part of your analysis **is** correct: CC-BY-SA permits me to take a high-res image you give me, modify it, reduce it to low res, and refuse to share the modified high-res image with anyone.