I'm no expert myself, but I do know that extensions of any type in general (music, pictures, documents) come with their own set of data that can be used to variety of manners, for example, a picture might have things like date it was created, last time it was modified, comments and other tags that lets other programs identify certain aspects of said specific file, that's "metadata". Since the game doesn't need any of that, removing them doesn't do any harm afaik. Unfortunately this is about as much as I know and I'm not even sure if that's right or if it's all there is to it.
As for compression, just think of the difference between a .wav file vs .flac one. They're both the same quality (lossless) but flac has largely less file size at the cost of slightly more complex decoding (CPU usage). PNG simiraly can be lossless but it depends on how it was compressed and which algorithm was used to do so. If you compress it once more with a more robust level of complexity, then the file size can be further minimized with no loss to quality, because what you're changing is not the quality of the file, but of the
compression itself.
I noticed this while trying to switch the animated PNGs to APNG. Even if the conversion is succesful, it unfortunately doesn't play in-game. Sad but what can we do.
Sure, though my internet speed is relatively slow so although I'm doing it myself it'll take a while just to upload the whole thing. Keep in mind that I'm doing the lossy version first so if you don't want to wait for the lossless one, it's pretty easy to do this yourself: just download a program that can correctly compress in batches without messing up the quality of the files. I recommend
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since it makes it easy to do it in just a few clicks. Here's a screenshot with the options you want to make sure no quality is lost should you want to test and see if it works. Just keep in mind it'll take a while to compress them all, there's over 7k images after all.
Oh right. If you try to compress the whole folder it won't work, you need to convert folders without subfolders, so don't start with the pic folder in content, start with the "bg", then "buttons", etc. Another one is "girls" that have subfolders, so go in and start with "body" then "face" as you go along. If you get any issues with the program do tell me I'll try to see why that is.
I'm already working on my own so once I finish with that I'll try to upload it here so everyone else can test them as well. As for credits, well, we can talk about it later, I want to see if this works properly first.