You have to actively replace the standard-images in /wp-content/plugins/lightbox-gallery/images/ with the ones e.g. supplied in the subfolder "de_DE" or your own images.
"Image x of x"-Phrase:
It seems as if this is not covered by the standard-language-files.
Instead, go here:
Line 38 ("Configuration related to texts in caption") and alter it the way you want it to appear:
// Configuration related to texts in caption. For example: Image 2 of 8. You can alter either "Image" and "of" texts.
txtImage: 'Bild', // (string) Specify text "Image"
txtOf: 'von', // (string) Specify text "of"
Hope this helps.