How to manage the media library between main blog and sub blogs?

I am using the global Tags in WordpressMU to enable my subdomain blogs to post into the parent (main blog). I am using images as part of the post, which means it gets stored in the sub domain media libary.
Now the problem, if this posts gets "propagated" to the main blog the post is there, it shows the palce holder (as I post some meta data as well) but the image is not shown as the main blog can't access the sub domain media library.
Can anyone help me in solving this issue, either with a piece of hacking or an plugin?
Thanks
Helmut