Where did i say that you should unzip the jar??? I told you about a solution that you can place the "new images" in your projects files-folder in a subfolder ftypes and the library will load them from there if they exists. THAT´s what I understand from Margrets post...
You can NOT add files to this jar i think. The Librarys Author have build in a function to use images like i stated above.
You need to use THIS possibility OR you write the author and ask him whether if he can put more images to this library per default or give you the soource and you build a new library from that with new images.
I think the best way is to follow Margrets way... Placing new images into subfolder of Files folder and then use the new images.