Quite often Prestashop users encounter problems with managing images. This happens because they do not fully understand how exactly this CMS works with images. For example, one of our forum users got the following problem: when he disables the watermark module, watermark images still remain. The thing is that for each picture the system generates a set of different images of different sizes and all this is administered under the following path: Preferences->Images
So, once you apply any changes in the watermark module, each time you need to regenerate all those images too.