NextGEN gallery lightbox - resize only images used in lightbox

I use NextGEN gallery with lightbox effect, but if I upload really big images (which is necessary) it loads forever. Is there any way how to set the lightboxed image to f. ex. to max width: 600px? Without resizing the source image (which can be still downloaded)? I have tried all of possible Lightbox Effect setting, but nothing has helped.

We use big pictures (+3000px width) because our users can download them through NextGEN Download Gallery so we don't want to resize them.

I have read throught this topic but it didn't worked for me:

- auto-resize-gallery-based-on-browser-size-or-scrn-resolution
- nextgen-problem-resize-images-automatically-in-screen-resolution-function

Thanks a lot for your help!