It's simple. Just 4 steps:
- Save a copy of the file as ODS (OpenOffice.org Spreadsheet).
- Change the filetype of the copy to ZIP (for example rename File.xls to File.zip).
- Open the ZIP.
- In the directory called Pictures will be the images, just as in the original.
For newser versions, it works also to save it as xlsx and rename to zip later. Extract it and in the embedded folder you'll find a mediafolder containing your pictures.