No - this is not permissions question.
Imagine - how you insert image via web browser? And IWP is the same.
On server computer you insert via FileMaker, isn't it? But on client machines - via web browser.
You ned to upload pictures (you want to insert) to server.
This is a bit complicated.
Via PHP API it is a bit easier.
But i general I have more experience with PHP API like IWP (take a look here - hierarchy.lv)