I'm having issues with the with the watermarking in the media gallery as the watermark gets pixilated and is of poor quality once on the image. The watermark process works just fine, I use a 600 pixels .png watermark, kept the original setting at 20x15. Tried various watermark images sizes to see if it would help but the problem remains. Am I missing something? Thanks for the help.
You should never manually manipulate the database values directly. It's possible that by doing so you may have permanently marked the original versions of the image (which we store without a watermark, deliberately for this purpose) and there won't be a way to recover from that without restoring from a backup.
If you ever need to change the watermark on existing images, you just do so by running the rebuild.
The watermark cannot be added automatically to existing images unless they have been watermarked before. To watermark images that currently don't have a watermark you need to do it manually via the "Add Watermark" button for each image, or use the inline moderation tools.