Hello,
This kind of issue can be link to watermark and product image characteristics :
- Format => Preferer png format
- Resolution => check this point in order to have a least 90 ppi or maybe more if your watermarl size is important.
- Superposition => your product image resolution is too different from your watermark (one with 72 ppi and other is 130 ppi for example), as results your watermark highlighted product image pixels.
If none this check able you to increase your image quality, can you provide your watermark AND your product image in order to process some tests ?
Awaiting news from you
Regards.