I came here to ask the same question, as the resizing did not seem to work on NextGen Galleries. Now I know.
To the OP. You can use NextGen itself to resize your existing images, just do the following:
– Navigate to ‘Manage Galleries’.
– Select a Gallery.
– Tick some image check boxes (or select all).
– Use the ‘Bulk Actions’ drop down to select ‘Resize images’.
– Hit ‘Apply’
– Enter your new desired size.
