Hi,

First of all, make sure your picture files are not BMP files, since
they are not compressed. If they are BMPs, then convert them to JPG and
see if the spreadsheet's size changed. If the picture files are already
JPGs (or other compressed formats) then i really don't know.
One other thing...in Windows size matters ;) check the spreadsheet
properties, it's size and it's size on disk.

I hope it's of any help,

Cheers.