Issue 1529, programmatic support of “Fixed Images”, has been fixed. As of December 2015, Google Apps Script can manipulate
PositionedImage objects in Google Docs. Previously, we only had control over
InlineImages via our scripts, so this is good news.
There are some hiccups with the introduction of this new feature, though, so this article aims to help programmers get quickly acquainted with it.
Note: This post is information I have concurrently posted on Stack Overflow.