How to delete unused images from PDF?

Don't know for sure, but here are some things you might try:

FindImages() / ReplaceImage(). Might be able to replace a big unused image with a tiny one to save space.

DeleteLayer(). Might be able to delete a layer with images, assuming they are on a layer by themselves.

CheckObjects(). Documentation just says it checks all objects to see if they're valid. Doesn't say whether it removes invalid objects, but may be worth a try.

I'm still amazed at the number of functions in this library, and haven't used them all by any means.

Good to know.

Meanwhile, here's a tool than definitely can remove images:

http://www.windjack.com/products/pdfcanopener.html

Unfortunately, it also requires Acrobat, so is pretty much out of my price range...

Have you try an extractpages function after the replacement to create a new Pdf file ?

It's also possible to do this manually, if you're not shy about editing PDF files directly. You'll need the free pdftk tool.

Here's an example:

http://www.savefile.com/files.php?fid=1846963

1. Uncompress the original PDF using pdftk:
    pdftk png_example.pdf output step1.pdf uncompress

2. Edit step1.pdf (using commandline edit.exe):
    Find the object of interest, in this case 5 0 obj.
    Remove all references to this object by deleting the
    lines associated with it, including:
      /XOBJ7BC610 5 0 R
      /XOBJ7BC610 Do
      Everything between the line starting with 5 0 obj
      and endobj
    Save the edited file to step2.pdf.

3. The xref table in step2.pdf is now corrupt, so we need to fix it. We'll also recompress the PDF:

    pdftk step2.pdf output step3.pdf compress

The final file, step3.pdf, is now a valid, compressed PDF, without the unwanted image (we left the text description below the spot where the image was). We can now use QuickPDF to add a new image in its place.