Quicksave only used the first slot and will always have the current image when quicksave is used. The next time quicksave is used it will again save it to slot 1 and overwrite what is already saved.
If you use "savegame" command then the save game dialog box will pop up and the player can then select their own slot. However, I wouldn't use that as a command until the next beta comes out. All you have to do is put in the instructions to the player that slot 1 is reserved for quick saves. You would then put a hud on the screen telling them the game was saved.