My tool (PDFSyntax[1], mentioned in this thread) is a Python library that is able to both inspect and transform PDF files.
Depending on your transformation use case, you may write an incremental update with only a few bytes at the end of the original file instead of rewriting it entirely. To my knowledge this feature of the PDF specification is often overlooked and not a lot of libraries implements it.
It is a work in progress and I have not developed functions for images yet, though.