Does phpwebgallery_images.MYI even exist in /var/lib/mysql directory? You can try to repair the table but from the sounds of the error, you're probably best off to reload from a backup. No backups and repair doesn't work you say, well, looks like you'll be redoing your gallery and after you're done, setup a good backup plan for the database..
You can try to repair a table with the following SQL:
mysql> REPAIR TABLE phpwebgallery_images;
That's if the table name is called phpwebgallery_images