Google Apps Script function to retrieve a list of all PositionedImages in a document. See https://mogsdad.wordpress.com.

getAllPositionedImages.gs

/**
 * Get a list of all PositionedImages in a document.
 * From: gist.github.com/mogsdad/600bbb24ecb360da7d8c
 * See: stackoverflow.com/a/20661113/1677912
 *
 * @param {String} docId         (optional) ID of document to scan
 *
 * @returns {PositionedImage[]}  Array of PositionedImages in document
 */
function getAllPositionedImages( docId ) {
  // Open document if given ID, otherwise use active document.
  if (docId) {
    var doc = DocumentApp.openById(docId);
  }
  else {
    doc = DocumentApp.getActiveDocument();
  }

  // Get handle on document's body
  var body = doc.getBody();

  // array to hold all images in document
  var allPositionedImages = [];

  var numElems = body.getNumChildren();

  for (var childIndex=0; childIndex<numElems; childIndex++) {
    var child = body.getChild(childIndex);
    switch ( child.getType() ) {
      case DocumentApp.ElementType.PARAGRAPH:
        var container = child.asParagraph();
        break;
      case DocumentApp.ElementType.LIST_ITEM:
        container = child.asListItem();
        break;

      default:
        // Skip elements that can't contain PositionedImages.
        continue;
    }
    // Collect images from current container
    var imagesHere = container.getPositionedImages();
    allPositionedImages = allPositionedImages.concat(imagesHere);        
  }
  return allPositionedImages;
}