Embedding PNGs in an app to create a Texture2D on runtime in iOS

Hi, I work on an app for kids that will require lots (500+) of high res (512x512 and above) sprites. The images are mostly lineart or clipart kind of images and they compress great to PNG (<25kb each in most cases) . All works like charm on Android but the iOS binary is a giant one. This is the result of my PNGs being converted. None of the compressed formats works for me. What I look for is an option to create a Texture2D on the fly - I will only need to create a couple of them each time so the performance impact should be neglible. Any ideas how to proceed? Thanks in advance.