Hi Igor,
If you were previously passing in a file system path ie using Server.MapPath before passing in the path, you need to change it to pass in a virtual path like ~/path/to/your/file.png or /path/to/your/file.png may also work, ie I'm not sure the ~ is required, but it can't be an already resolved path for the disk file system, otherwise it would not work for other file systems with different storage. The default filesystem plugin is the disk file system, but it will do the .MapPath internally.
Hope that helps,
Joe