The user supplied data for the Alt & Title Text field isn't escaped before being printed out in the value field:

Vulnerability

from nextcellent-gallery-nextgen-legacy/admin/manage-images.php lines:
503 <td <?php echo $attributes ? >>
504 <input placeholder=" <?php _e("Alt & title text",'nggallery'); ?>" name="alttext[<?php echo $pid ?>]" type="text" style="width:95%; margin-bottom: 2px;" value="<?php echo stripslashes($picture->alttext) ?>"
505 <textarea placeholder="<?php _e("Description",'nggallery'); ?>" name="description[<?php echo $pid ?>]" style="width:95%; margin: 1px;" rows="2" ><?php echo stripslashes($picture->description) ?></textarea>
506 </td>

The HTML code produced is:

<td class='alt_title_desc column-alt_title_desc'> <input placeholder="Alt & title text!" name="alttext[1]" type="text" style="width:95%; margin-bottom: 2px;" value=""><script>alert('hi')</script>"<" /><br/> <textarea placeholder="Description" name="description[1]" style="width:95%; margin: 1px;" rows="2" >"</a><script>alert('hi')</script><a>"</textarea> </td>
<td class='tags column-tags'><textarea placeholder="Separated by commas"name="tags[1]" style="width:95%;" rows="2"></textarea></td> <td class='exclude column-exclude'><input name="exclude[1]" type="checkbox" value="1" /></td>

ScreenShot: