Maybe not much help, but check out the sdk at sdk.episerver.com. There you will find a developer's guide which talks about the file system. 

But basically you get the file like this:

System.Web.Hosting.HostingEnvironment.VirtualPathProvider.GetFile(path)

And the physical structure is because you are using the versioning provider. It stores the files as guids and uses the guids three first pairs for folder structure. So the file 77E21AEE-9895-11E0-9B8A-206F4824019B.jpg will be stored in the folder stucture: 77/E2/21/. Hope that makes sense, otherwise I do believe that the sdk talks about this. 

you can get it as follows

UnifiedFile uf = HostingEnvironment.VirtualPathProvider.GetFile(virtualPath) as UnifiedFile;
string localPhotoPath = uf.LocalPath;

make sure you import

using EPiServer.Web.Hosting;