Here's one scenario:
Insert the picture into a global container field that does not have external storage specified as the means to select the picture for a new record. Use a script to:
a) create the new record and thus generate the new record's ID
b) use set variable to compute the path to the computer's temporary folder and specify the needed file name (recordID).
c) use export field contents to export a copy of the file using this path variable to both export to the temporary folder and to rename the file.
c) Use the same variable to re-insert the file from the temporary folder, but now insert it into the new record's external storage enabled container field.
Thank for this Phil.
Can you help me on track with point b? I think I have to use Get, Let & Position, but I have no clue on how to achieve this.