For those of you who would like to add, not only picture "title" but also, picture "captions" in the lightbox view of the image...

It's a bit fastidious but it works...
You must edit 2 files in the the image store plugin folder :

_inc (folder) > store.php
_js (folder) > touch.jquery.js

First lets edit store.php :

around line 1375 (in "/* Get image tag" paragraph), find this :
 $output .= '<a data-id="' . $enc . '" href="'. $link . '" class="url fn" title="' . esc_attr( $data['title'] ) . '" rel="enclosure">'; 

and modifiy to something like this :
 $output .= '<a data-id="' . $enc . '" href="'. $link . '" class="url fn" title="' . esc_attr( $data['title'] ) . '" subtitle="' . esc_attr( $data['subtitle'] ) . '" rel="enclosure">'; 

then, around line 1587 (in "/** Display galleries" paragraph), underneath  $title = get_the_title( $image->ID ); 
paste this :  $subtitle = apply_filters( 'ims_image_caption', $image->post_excerpt, $image ); 

finally still in store php around line 1603 (still in "/** Display galleries" paragraph), find this :
 $image->meta += array( 'link' => $link, 'alt' => $title, 'caption' => wptexturize( $title ), 'class' => $classes, 'title' => $title); 

and modifiy to something like this : 
 $image->meta += array( 'link' => $link, 'alt' => $title, 'caption' => wptexturize( $title ), 'class' => $classes, 'title' => $title, 'subtitle' => $subtitle); 

OK : save your store.php file

Now we must make the caption appear in the lightbox :
let's edit touch.jquery.js

underneath (around line 93 if you clean the code, I use http://www.dirtymarkup.com/)  title = e(".ims-touch-" + t).attr("title"); 
paste this :  subtitle = e(".ims-touch-" + t).attr("subtitle"); 

then around line 95
find this  e("#ims-slide-" + t).html(this).append(e(".ims-touch-" + t).next(".img-metadata").clone().prepend('<span class="image-title">' + title + "</span>")) 

and modify to something like this : 
 e("#ims-slide-" + t).html(this).append(e(".ims-touch-" + t).next(".img-metadata").clone().prepend('<div class="image-subtitle">' + subtitle + '</div>'+'<div class="image-title">' + title + '</div>'))  (you can then modifiy .image-subtitle in style.css)

save touch.jquery.js

Done...
Don't forget to put some captions in the admin>galleries> "images title/caption" to see the result...