It seems there is no option to force some scaling so images can be displayed correctly also on smaller screens.
Wrong. Check the options page for Shrink large images to fit smaller screens
To have an image displayed not bigger than 70% of the screen (width or height) how can this be achieved with jQuery Lightbox?
Unless you edit the javascript, you can’t. The plugin is meant to maximize viewing area within a given display surface. If you want to arbitrarily limit the viewing size, upload smaller images.
Btw.. there is a scaling option but no value can be set eg. 80%, 70% or something?
It’s a pixel margin value. Ergo: just enter the number. This setting is only there to enforce a “gutter” between the lightbox and the edge of the display, for users who prefer to click the greyed-out background to close an image rather than using the “close”-button inside the lightbox.
