Check if MTL PART# is a text field or a number field ( or the result type of a calculation ).
Also use an underscore instead of a space in MTL PART#.
MTL PART# is a text field, auto calculation
Sounds like the file path to each file differs too much for the approach you are specifying to work. (And you posted a calculation not a script.)
I'd insert the pictures into container fields using either the "by reference" option or external storage depending on which approach works best after taking all tasks that you need to perform with these image files into consideration.
This either stores the reference (file path) to each file in a container field or copies the files to a special storage location and then inserts the path to that location into the container field.