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I see you're encoding an element with the key "Picture", but trying to extract it with the key "Picture.jpg". It looks like you meant to decode it like this:
Base64Decode ( JSONGetElement ( Get ( ScriptParameter ) ; "Picture" ) ; "Picture.jpg" )
Aside from that, FileMaker container fields are finicky about matching the file extension to the actual data type. The Base64Decode function really wants that file name with the right extension for the data type. It can be worth double-checking that the file type matches, or even passing the container file name along with the data. FM-JSON-Types handles this by sending containers as a child object with separate elements for base 64 and file name.