Ad networks tend to limit maximum size to as low as 80KB or 150KB so minimizing image sizes is crucial. By minimizing images you achieve
By reducing load time the ad visibility time increares and visitor are more likely to see it. By improving runtime performance user tend to spend more time in the game. And naturally, everbody likes good user experience.
This step can reduce PNG size up to 90%, and can normally be done without losing anything in quality. Here are some programs which can be used to perform this step.