Bigger Images are Better Images
When looking for best practices to optimize images for the WordPress (or the web in general), I stumbled across a lot of articles along the lines of "Mistake #1: We uploaded gigantic images" or "I use Photoshop […] with the Save For Web option". These are just two of a plethora of post, basically saying: Optimize your images "offline", only upload the size you need/use (today). The ultimate goal we all are trying to achieve is the same: Optimized images. Our images should have the smallest possible file size while maintaining the best quality. The way of getting there is completely different though.
Disk space (regardless of whether it is offline, "in the cloud" or on your server) is available in vast abundance. Bandwidth, data quotas and impatience of the user (me included) are what limits us. That’s why I always recommend to upload the full-size image including all metadata and let the server decide what image a user gets served. We never know how our images will be displayed tomorrow. Be it because we change the look/theme of our website or because technology advances and pixel densities become higher.
A lot of times when I recommend this, people tell me "But I always have the original on my computer". There are several downsides to this however: If you really were to re-upload the images you’ll have to change each and every one of them and possibly go through all of your content. This is usually something we can’t afford to do (and won’t for that matter). Another disadvantage is that we either lose the original among the way (while upgrading computers for example) or have images all over the place on several backup disks and it would take ages to find the correct ones. If the original images are on your server you have one SPOT (single point of truth) and that is on your web server (and hopefully a backup).
Images should have the smallest possible file size while maintaining the best quality
When Matt Cromwell suggest "No really, Don’t Upload that 3MB Image to Your Website Straight from Your Camera" my answer would be: "Oh yes do exactly that." Keep as much data as possible on your server, you never know when you’ll need it.