Wow, again, where to start... There are so many articles out there on the subject of images and media for WordPress. Most of the top results on Google have good information, are well presented, and even have some tidbits of knowledge for intermediate and advanced users. But there's just so little information that falls into the 'revelation' category. You know, that bit of information that turns on the light bulb in a single instant, changing something from knowledge into working, functional understanding.
Here are some notes on the a fore mentioned 'knowledge epiphany' in the form of notes to myself (hopefully later to be developed into a full blown knowledge base type of article).
WordPress, Images, Efficiency, and Viewing Pleasure
I don't know about you, but I come from a world of high speed internet and large, abundant screen real estate. I sit here writing this today in front of a single 30 inch monitor (along with 5 other 24 inch monitors spread out in an array around said 30 inch monitor). I don't like staring at tiny little images that have foggy detail. You know what I'm talking about. It ranges from that small detail on a product you want to buy and can't see in the image to some historical photograph that's missing the pop and wow factor because someone downsized it so much. Come on! Fast internet, big screens, why not present the biggest image possible to a viewer? Oh, right...
Small Screens
Forgot about all those portable devices that are hooked up to McScrooge 'unlimited service plans' (in that there's no limit to the amount of money they're willing to charge you) from cellular phone companies. For those devices, because of the small screens, they don't need great big pictures. Sometimes that's just the circumstance. Not to get off on a tangent, but it's worth mentioning that I just do not understand why anyone would want to watch, view, read, etc. something on a phone or even a tablet device, if they could do the same thing with a real computer and a great big screen. OK, I get it, sometimes it isn't possible to view something on a great big screen. But it's those times that a big screen is available and someone would still choose a tiny little, super-duper, Retina display from Apple over a cheap 50 inch TV from Wal-Mart. Why? OK, back to the subject.
History
WordPress Thumbnails. Errr, don't you mean 'Alternate Image Sizes'? Well, yes. But WordPress life started out referring to them as 'thumbnails'. A thumbnail being a downsized (as in dimensionally and quality) to a smaller size, which in turn usually represented a link to a larger, full sized image. That's how it began with WordPress. And sort of like Max Headroom, the name just sort of stuck, even though it doesn't refer to the same thing anymore.