Take original image and resize it to max of 1024x1024:
convert img/original/image.jpg -resize "1024x1024>" img/large/image.jpg
Take resized image and create 200x200 thumbnail:
convert img/large/image.jpg -gravity Center -thumbnail 200x200^ -extent 200x200 img/thumb/image.jpg
update _includes/gallery.html with new image.