I suggest you try a test upgrade to Tiki6 LTS. You may be getting an issue because your old Tiki (which was designed for PHP4) is on a more recent version of PHP.
You can do a direct upgrade. Tiki 6.x contains all the SQL instructions to upgrade from 1.8.x or any later version.
- Make a backup of your current site
- Install a Tiki 6.x and point to the DB backup
You have the option to store images on the file system instead of the database. This will solve the issue with uploading larger images. There is also a "mover" feature to go from filesystem to database or vice versa. See: tiki-admin.php?page=gal
In terms of image gallery vs file gallery: you can continue to use image galleries. There is not much dev on them, but they will only be removed when an upgrade script is available. No one has volunteered to take this on yet, and we need to migrate the data and make sure URLs still work.
About tiki-show_all_images.php, that was experimental and AFAIK, still is. There has not been any work on it in years:
Please keep us updated with your progress.