I haven't tested this but if you take and add all the images I one site then in the site manager for each of the sites set the Settings>System>Files>Storage fields to the same place. Then when you go to select some image from a media library you can select a site and choose the library.

If you leave the permissions alone you could also use the "GetMedia" link and get the media without even navigating to the media library if you have the media's GUID. It would simply access the file system and display the image.

Hello.

I believe it is.

First, you need to disable "Use site-specific subfolders for custom media libraries folder" setting in the Site Manager / Settings / Content / Media file. Then, you need to choose some "Media libraries folder".

Finally, you just need to ensure that two or more media libraries, no matter what site they belong to, share the same "Folder name."

Does it help?

Best Regards,
Radek Macalik