You should be able to do that making a symbolic link.

ln -s target_filename symlink_filename

target_filename would be /var/path/to/domainb.com/images

symlink_filename would be /var/pth/to/domaina.com/images

So, this should do the trick:

Code
ln -s /var/path/to/domainb.com/images /var/pth/to/domaina.com/images