For a gallery download with Wget made by NextGen Gallery (at least this is what I read from then nggpage=2 query string), I had success with:
wget -nc -nd -A '*.jpg' -R 'thumbs_*,index.html*' \
-I /wp-content/gallery/ -r -l 1 \
http://www.example.com/category/2014/04/08/slug/
My example gallery with with all jpeg files. The switches are documented in the GNU Wget Manual.
hakre on wordpress 
