Changing the 'Thumbnail size' in the WordPress Dashboard will not regenerate new thumbnails for any images which have already been uploaded but will create thumbnails at the new dimensions for subsequent uploads so you would have to re-upload the images for your gallery for the change to take effect.
I notice that your gallery currently uses the same images for both the thumbnails and the main images. The thumbnails are now scaled down and look noticeably sharper than they did previously. If you do not see a difference, try clearing your browser's cache before reloading the gallery to ensure that your browser is not hanging onto and using older versions of your gallery files.
When uploading an image to the WordPress Media Library, WordPress creates several different-sized images.
WP-SimpleViewer uses the smallest image for the thumbnails in the gallery. If you would like to change this, then open the gallery's 'wp-simpleviewer/gallery.php' file in a plain text editor and edit line 58 and change the word 'thumbnail'.
$thumbnail = wp_get_attachment_image_src($attachment->ID, 'thumbnail');
Possible values are:
thumbnail - default 150px x 150px max
medium - default 300px x 300px max
large - default 640px x 640px max
full - original size uploaded
Please note that the line number quotes above refers to the current version of WP-SimpleViewer (v18.104.22.168).
SimpleViewer Support Team