Thumbnails and low-resolution pictures are generated on-the-fly by phpGraphy, that means if it doesn't already exists when a user is requesting it, phpGraphy will generate it. You may want to generate them all in advance (ie: for performance issues), this can be accomplished by several ways:
When logged-in as admin, using the Batch Processing menu.
From the command line or from a crontab, you can call the script with the batch_action=generateall as argument (For this to work you need php-cli to be installed).
/yourpath_to_phpgraphy/index.php batch_action=generateall