Wait, are you just renaming the files and overwriting the existing ones instead of adding your new images to the appropriate .js files? That's probably why it isn't working. If you have a file called "X.mp4" and replace it with a webm file, then you'd have to change the extension to .mp4 to get the game to call it, and even then it probably won't work because it's calling a .mp4 file and you're handing it a .webm file.
You should look at the guide on the game's website to learn how to properly make an image pack. You need to put the new images in preferably a new subfolder, then add the filenames to the correct entries in the appropriate .js files so the game uses them.