Your thumbnails are burred because they are having to be upscaled by SimpleViewer as you have specified thumbWidth="75" and thumbHeight="75" in your gallery but the actual images are only 50px x 50px.
You could either:
(1) Increase the size of the thumbnail images generated by WordPress by going to 'Settings -> Media' (the default values are 150px x 150px).
As you no longer have the original images, you could download the largest available copies of these images from your 'home/wp-content/uploads/2013/04/' folder (using an FTP program) and then re-upload within WordPress to generate new larger thumbnail images.
(2) You could tell the WP-SimpleViewer to use the next size up (rather than your 50px x 50px thumbnails) by editing the plugin's 'wp-simpleviewer/gallery.php' file in a plain text editor and changing line 58 from:
$thumbnail = wp_get_attachment_image_src($attachment->ID, 'thumbnail');
$thumbnail = wp_get_attachment_image_src($attachment->ID, 'medium');
(The line number above refers to WP-SimpleViewer v22.214.171.124.)
SimpleViewer Support Team