This is a real world iOS example, but can be applied to PNGs in any scenario.
In iOS the splash screen images have to be PNGs. This works great for simple images with just a few colors, but will result in huge PNGs if you plan on having photo content, especially on the retina iPad with a screen resolution of 2048×1536.
At first I figured because PNGs are lossless that there’d be very few ways to reduce the filesize. I am going to run through the well known ways and some others that were new to me.