At this point, you may hopefully be asking yourself, "What about mobile devices?"—it's a good question after all: the whole point of responsive design is to cater to mobile devices! Most of the tips and tricks we will encounter throughout this chapter will work on a mobile platform, although there are some useful guidelines that are worth noting:
Shrink the images. Use any online service such as TinyPNG or XnConvert; they need to be made as small as possible without sacrificing too much quality.
Be careful with retina images. Memory usage for these can vary dramatically between different mobile devices, so ensure you set your media queries appropriately.
If you're using jQuery to provide fallback support, then consider using conditional loading to only call jQuery when needed and not by default.
Make sure that different sized versions of the same image have been created. There is no point in forcing a mobile user to download a huge file when a small one will do! To get help with this, look online for tools such as Andi Smith's Responsive Images tool at http://www.andismith.com/grunt-responsive-images/, which can help automate the process.
Test in an online applet such as the Responsive tool available at http://coolestguidesontheplanet.com/responsive/, as there is no substitute for testing! It is much better to test thoroughly now and fix errors before going live than to suffer the embarrassment at a later date.
Work in a mobile first capacity. Mobile devices need to be set to download the smallest images first; browsers will handle the replacement with larger images automatically if the media queries have been created within the site.
Consider using something similar to the Network API (http://code.tutsplus.com/tutorials/html5-network-information-api--cms-21598) or Modernizr (http://www.modernizr.com) to determine whether the visitor is using a mobile platform; we can link these to jQuery/JavaScript based media queries if required to determine which image should be served.
Don't use display: none to hide images, serve them in media queries. Using the former approach still downloads them even if they are hidden.
Tip
Remember this sentence from RevUnit's Seth Waite: "1 in 4 people abandon a web page that takes more than four seconds to load." This becomes more critical with mobile devices; loading a large image will blow this straight out of the water! You can see his original article at http://sethwaite.com/2012/08/how-slow-website-speed-destroys-your-conversion-rates/.
Now that the theory is out of the way, let's get coding! We're going to begin with one of the key elements of working with responsive media—catering to high-resolution (hi-res) or retina-based images.