The functionality to export or copy the graphs as images lies in the Excel application. You're best bet is to extract the images on a Windows machine. If that is absolutely not possible, then try the OpenOffice.org API.

Depending how many charts there are, and if you have access to a Windows machine, it might actually be faster to just copy them all to the clipboard, one by one, and paste them into Paint or Photoshop or IrfanView or whatever. This works quite well.

Good luck!

--marmot