I found an answer elsewhere, namely the iText mailing list.
The following code works for me – please note that I switched to PdfBox:
PDDocument document = null;
document = PDDocument.load(inFile);
List pages = document.getDocumentCatalog().getAllPages();
Iterator iter = pages.iterator();
while (iter.hasNext()) {
PDPage page = (PDPage) iter.next();
PDResources resources = page.getResources();
Map pageImages = resources.getImages();
if (pageImages != null) {
Iterator imageIter = pageImages.keySet().iterator();
while (imageIter.hasNext()) {
String key = (String) imageIter.next();
PDXObjectImage image = (PDXObjectImage) pageImages.get(key);
image.write2OutputStream(/* some output stream */);
}
}
}