The problem is this: if the high quality zoom option is enabled, white backgrounds in photos appear with a grid texture, made of grey pixels.
Here is a portion of a photo that perhaps explains it better (left - grid vs right - white):

Disabling that option solves the problem, but also gives more rough images of course.
I've checked other programs and they don't seem affected by this issue (Faststone image viewer for example, which also has a similar option, isn't affected by the issue either turning this option on or off, the windows viewer isn't affected, nor irfanview, etc).
Could you please take a look at it?
Thanks.
PS: this is the option I mean:
