Hi, Ben,
I don't know if your fabulous pic 'dad&carmelle' is one that suffers this affliction, but I looked into it. The image to which you linked is 800 x 533 px.
Of course its onscreen pixel dimensions when I view it depends on the magnification to which my browser is set.
When I set my browser for the default magnification (which I don't usually do, especially for OPF, since it makes the text too small), it is 800 x 533 onscreen.
I don't find that "OPF" does anything to the images. It seem to deliver them to the browser as is. There is nothing in the incoming HTML code that calls for any "resizing" in the browser:
<div align="center"><img src="http://www.studio-beni.net/dad&carmelle.jpg" border="0" alt="" /></div><br />
Of course the browser won't necessarily put them on the screen at 1:1 (depending on what you tell it to do, magnification-wise).
It may be that the OPF style imposes a rather small font size. It may be that this leads you to set your browser to a high magnification on the OPF site, which may be the cause of the phenomenon you mention.
Best regards,
Doug