Seperate Drupal instance as files server for other Drupal instances My task is to create files gallery which is files library for other Drupal instances.

Other instances can upload files to library and browse them. 
File metadata are stored in library (A) and client, but physical file is stored only in A.
So instance A acts like some kind of CDN.

Could someone share with me his thoughts about this problem?
Which modules to use? (besides CDN which I am testing now)

I can accept only free solutions.
You could use feeds module to periodically import file URIs from XML on your file server to you individual drupal instances. You might then be able to hook that up with the media module and use the files on all your different sites. 

Edit:
If you're just looking for a cdn for your images you could probably just use Amazon S3. Looks like there's a few modules already built to work with it