Do you have any idea how I can do this?
This is an example from the galleries in ://www.mach2boats.com/index.php/en/media-a-images/regatta-photos/2012-victorian-state-championships
The original images were uploaded as high res, but that's not the res they show
The image size in the Galleries Manager is 900x500px
The Gallery configuration only has size for the thumbnails.
I tried adjusting the sizes, but didn't seem to alter on the resolution.
Yes but I guess you didn't realise that you can actually put any dimension in those boxes. What ever you put there, RokGallery will try to reduce the dimensions of uploaded image to that specified dimension