Different thumbnail to different full sized image

I want to use a set of thumbnail images that are different to my full size images.

Is it possible to do that with Nextgen Gallery? So far I haven’t managed to work out how to do that…. only crop parts of the bigger image, which isn’t what I want.

A working example of what I am trying to achieve can be seen here – https://www.juliemontagu.com/press.html

Any help would be greatly appreciated.

Thanks!

https://www.ads-software.com/plugins/nextgen-gallery/