Hi,
I've updated my Zenphoto to the latest version but seems that the "image_allow_upscale" feature is missing.

In my old ZenPhoto I had this configuration option:
// Allow images to be scaled up to the requested size? This could result in loss
// of quality, so it's off by default.
$conf['image_allow_upscale'] = false;

Is it possible to obtain that feature with the latest version of ZenPhoto?

Did you look at the image options tab in admin?