chaging gallery image max-height to 90% of viewport

90vh = 90% of viewport height. 650px is the fallback for [browsers that do not support the vh unit](http://caniuse.com/#feat=viewport-units). assuming for the most part that older browsers are likely to be running on older computers and therefore lower resolution monitors. 650px seems appropriate for a 768px height display.
To make this faster you can pass the parent gallery node as an argument to imager here, that would scope the upgrade just to these images.