For a project recently I was sent some excel files with images embedded in them. Not fun. Then I discovered that these were in some random windows format of emf or wmf (depending on whether I exported as .xlsx or .ods from libreoffice) which I think was just wrapping a jpg/png file into some vector/clipart format. Fortunately there’s a great script called unoconv that uses bindings into libreoffice/openoffice to render pretty much anything, however it doesnt seem possible to change page size/resolution. If you use the PDF output though you can get the image simply embedded in the PDF, then use the pdfimages command to extract the original images out of there. Finally some of these had different white borders so I cropped these and converted to png. Full commands below:

rm -fr out; mkdir out;
for i in xl/media/image*.emf; do
  unoconv -f pdf -o t.pdf "$i";
  pdfimages t.pdf out;
  convert out-000.ppm -trim out/$(basename "$i").png;
done