I understand your concern but we all know the first ordinance: "What ever you do, do a fresh backup before doing it".
So please do a backup of your images (and tiki db).
It is possible Tiki corrupted image while uploading them but it is not possible for Tiki to corrupt them while just looking at them.
What I would do:
- Do backup
- Upgrade your Tiki to the last 18.2
- Upload an image and check
If all good, now your next step is to track the corrupted image and upload a new version.
Hope this help