" I would like to extract the longblobs with the images in question just to make sure I have them as JPEGs on disk..."
There is the "mover" feature to go from filesystem to database or vice versa. See: tiki-admin.php?page=gal
charset de-iso-8859-1: Tiki 6 has a UTF-8 converter in the installer. This will assist in converting. But do test everything thoroughly on backup data. Because of legacy concerns and variations of data from hosts, it's doesn't always work well.