Is there any way out to create additional thumbnails of custom size in nextgen gallery while uploading images?
For example I would like to create 3 thumbnails of size 25*30, 100*50, and 400*350 to display them in certain custom pages.Or can i add custom code to resize when image upload is triggered and if so where?
Make shure your theme function have
add_theme_support( 'post-thumbnails' );
to add a custom image size use
add_image_size( 'thumb-one', 25, 30 );
for more details have a looh here
If you want to write a custom function to re-size the thumbnails you need to have a look at
gd.thumbnail.inc.php in the lib folder of the plugin. This is where the thumbnails are generated.
Alternatively you can create custom a template for displaying different thumbnail sizes.
If you have a look in the view folder anyone of those files will give you a clue as to how to create a template for the plugin.
Once you create a template you can display it with a shortcode like
[ nggallery id=X template=your_template ]