Image asset protection/encryption in an Android app

Question

I have created an Android game where I use TextureAtlas generated from the TexturePacker tool. I have paid an artist to create good quality assets. So I have these nice large png files with alpha, containing all the assets, waiting to be stolen by anybody unzipping my apk. And I would like to protect them - at least a little - from hackers and thieves.

I have searched and found nothing except the usual "you will NEVER be able to fully protect your app" or "only thing you can do is to create you own cipher and encryption system from scratch by yourself" (yes, I am exaggerating a little here). I find these answers unseless for my case because: - I know it is impossible to make an app, a piece of code or an asset 100% protected from hacking. The goal here is to protect it a little to make it hard for 95% of hackers. - I know my assets are not the most beautiful piece of art ever made but I also know they are better than those of a majority of Android apps and any game cloning developper will see that.

Is there any way to remove the transparency from the png files but keep it in the game? Is there any way to add some kind of watermark that would be automatically removed in the app?

So thanks for sharing your thoughts