I am not the plugin designer by the way - I'm just trying to help you out.
I noticed you have two extra folders in your galleria-themes folder. This is what you have:
This is what you should have:
Move all of the individual files from the "twelve" folder and place them directly into the galleria-themes folder (really, you only need the js file, but go ahead and move everything so it's consistent)
Delete the (now empty) folders: themes and twelve.
Also, are you using the wordpress built-in gallery? You know, the one where you click "add featured image", then upload photos for that page or post? It will automatically generate the galleria when you add the photos to the wordpress gallery. Try it with just the basic galleria galleria plugin first to see if it works for you - then try adding the twelve theme. You should just be able to install the free galleria galleria plugin and have it work without any further effort on your part.
Confirm that it works by uploading your photos to the WP gallery, then add the shortcode [gallery] to your page or post where you want the photo gallery.