before you start pk2 editing, right click on the original Media.pk2 file and then click on Properties. you will get an window like this :

Sorry for the language, its swedish.
then take a look at the size , remember the size of the original Media.pk2.
thereafter you start editing it, make u have a backup lol
after you are finished with your editing , save the media file in your hexeditor then look at the size of Media.pk2 again, if it is different, take the backup of media.pk2 and put in the silkroad directory and overwrite the one u screwed up, or look after guides.
im not a pk2 pro so i only replace things that is exaclty the same lenght otherwise i use guides.
hope it helped // TwoKay