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.
When users download apps, it’s always a pain having to wait for a super long download. Getting them in as quick as possible should be a high priority. When an app download size is too big, I often find myself lock the phone and sometimes even forget to come back to it for a while. On the AppStore there’s even a threshold to prevent users download apps that are too big over 3G, currently at 50MB I believe.