Almost all the materials on this site are presented in DjVu format. Why don’t I use JPEG? The reason is file size. My project consists of thousands of maps from various sources. DjVu format allows to decrease the file size of the image in 1,5 - 3 times (comparing with JPEG), providing good quality.
To view the material from this site you can use either native DjVu plugin from Lizardtech company (freeware), or third-party browsers like WinDjView (freeware).