As long as I can remember, we have been trying to make digital photos smaller for faster on line uploading or when emailing them to friends.
For a long long time, JPEG is the default format that everyone use.
According to Wikipedia:
In computing, JPEG ( // jay-peg) is a commonly used method of lossy compression for digital photography (image). The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality.
I remember fondly, in the early days of internet; the fastest way to break a friendship was to email them a 2 to 3 MB picture ;) A sure way to stuff up their email "in box" too!
Now a day you can easily re-size your photos by compressing them on line, the last time I Google "resize photos"; I got SIX MILLION hits!
There are literally hundreds of FREE sites that you can upload your pictures and get back a resized copy within seconds!