You need to "mount" the server share - once you've done that, you'll be able to access it as though it were a local directory (this is more-or-less the *nix equivalent of mapping a network drive to a drive letter). To mount Windows shares, the CD uses Samba
There is a directory named /mnt which is traditionally used as a base for mounting network shares. System Rescue CD has had a bug in the past (don't know if it's fixed now) that meant you shouldn't mount things directly in /mnt, but in a sub-directory.
Create the directory where you will mount your Windows share. Open a terminal and type:
mkdir /mnt/win
Then, to mount the share, type something like:
mount -t smbfs -o "lfs,username=WindowsUser,password=WindowsPassword" //1.2.3.4/share/ /mnt/win/
or (if you don't need user/password):
mount -t smbfs -o lfs //1.2.3.4/share/ /mnt/win/
The "lfs" option enables large file support, which is needed to handle files larger than 2GB.
Once that is done, you can simply run partimage and set the image file to something like
/mnt/win/Windows_Image.img