Hi,
I really appreciate your effort in explaining how it works.

Let me ask in this way:
1) If I set the watermarking to yes for the three settings provided by the plugin should I see ALL images watermarked aywere they are published in the site?
If the answer is YES then there is something wrong if the answer is NO that ìs what I see, IE in the category product lisitng (tpl_product_list.php) the images are not watermarked.
2) If I just activate the IH without any change on the images set
  • without changing the settings of the images size in the zen cart image handler configuration
  • without any compression software installed in the server,

should google page speed give a better scoring for the site compare with IH not activated?
If yes then there is somenthing wrong, if NO then i is what I see but I do not understand if it is google that is fooling me or if they are actually detecting images not compressed enough, hence too big.

I hope I made it clear enough with my english far from being perfect, if it is not clear, be patient and try again, there are cances that I can understand after a while...
Ciao ciao