I would easily venture to say that optimizing your images so that they 1) not only look great, but 2) that they load as fast as possible which means they are compressed and displayed on the page at the correct size needed, and nothing more.
We are a Macintosh (feels good to say the entire name) based organization, so we heavily utilize ImageOptim and that helps do pretty much all of the heavy lifting when it comes to compressing file size without sacrificing the image in terms of quality.
Using the incorrect image format for its intended purpose is a simple rule I see all kinds of dev’s breaking these days. The majority of websites utilize a combination of JPEG, PNG, GIF.
Use images to your advantage and when needed, not just for the sake of filling areas with images. You want to make sure all the pictures throughout the website have an exact purpose, as to not overdue or cause to much clutter and distract potential clients from the very reason they are there in the first place.
Future image formats like Google’s WebP are all the rage and are now being adopted into all the browsers of the modern day, such as Google Chrome, Mozilla Firefox, and Microsoft Edge / Internet Explorer. Make sure, when available and more adopted, to transition to these modern-day formats with incredible quality and compression metrics.