There are two ways to add new sprites to the Astonia engine.
The first one (the easy one) requires you to repack an existing sprite, and include your own. Daniel Brockhaus has released the sprites which were used for the game (you can find them here. The easiest way is to add your sprites to the folders and repack them. For example, you could open the folder 00017000 and add sprites there (make sure you name them properly). AFter adding the sprites, you put the folder in your .../gfx/ folder, and run the client with the -gp 17 command (i.e. 00220000 would be -gp 220, 00070000 would be -gp 70 etc.). For this, just create shortcut and add -gp 17 after the target. If it is successfull, a new 00017000.pak will be created and it will include any sprites you added.
The second option you have, is to create a new .pak file. (to be continued).