When image uploaded to the wp gallery - thumbnails automatically created, in my case 294x294, 191x191, 99x99 - these thumbnails are generated at very high compression / low quality.
In order to improve the appearance of the products in my shop and to add some product description to the thumbnails, I Photoshopped my own set of 294x294, 191x191, 99x99 images and uploaded them directly to the server via ftp replacing automatically generated thumbnails.
But for some reason over time my thumbnails get replaced with automatically generated images again and again, probably due to some size threshold.
WP generated thumbnail, 7.5kb :
Required thumbnail, 14kb :
What script does automatic regeneration of already existing thumbnails and is there any way to disable it?