Responsive Images (different images in HTML for different situations) is an important one.
Few important bits :
-
Use of
srcsetattribute for switching between different versions of the same image.
http://responsiveimages.org is a well of resources on this subject. -
Use of automation tools for imaging – One of my favorites is Grunt and here is a nice read about it: http://addyosmani.com/blog/generate-multi-resolution-images-for-srcset-with-grunt/
It really makes a lot of difference when user on mobile opens a page with images that are sized for it, less data and well faster loads 🙂
Tools like Grunt may seem like they take some work to setup but once you start working with it its so easy and fast.
More reading material: