More precisely, it appears that usb-creator-gtk selects the valid ISO with the oldest file creation date. If this file is moved to a different folder, the next oldest date is selected. It also appears not to allow selection of ISOs in other than the original browse directory if a valid ISO exists. If there are no valid ISOs, then I am able to browse to a different directory and select any ISO, not just the oldest.
$ usb-creator-gtk -i <preferred iso image>.iso
issued from a command prompt will select and load the right ISO to create the USB startup, regardless of creation date
More precisely, it appears that usb-creator-gtk selects the valid ISO with the oldest file creation date. If this file is moved to a different folder, the next oldest date is selected. It also appears not to allow selection of ISOs in other than the original browse directory if a valid ISO exists. If there are no valid ISOs, then I am able to browse to a different directory and select any ISO, not just the oldest.
$ usb-creator-gtk -i <preferred iso image>.iso
issued from a command prompt will select and load the right ISO to create the USB startup, regardless of creation date