The advent of digital cameras has put digital photography in everyone's hand. The problem is that most users don't understand about picture sizes. The typical camera takes high-resolution picture which is great for printing but bad for web and email use. A picture straight off a camera weighs in at over 1 megabyte in file size. A typical web photo however should be no more than 50 - 60 kilobyte (1024 kilobyte = 1 megabyte). This is where resizing cropping and optimizing comes along.
A word about resizing - at the top of this screen is a copy of the pixel ruler I use when designing. If you resize your window, you will see the pointer move to show you what size window you are using. The photo I used for this example is 2563 pixels wide. The computer I am using right now has about 1200 pixel resolution, so I would only see about half the photo.
Photo Cropping - photo cropping is cutting out extraneous background from a photo to enhance what you want to show. For Example, the first photo below has been resized from a raw digital photo. The original was resized from a off-the-camera high resolution of 2563 x 1906 pixels. The resized photo is 450 pixels (wide). The second photo cuts out a lot of the unneeded detail and at the same 450 px size emphasizes more of the detail of the boat.
You should crop your photos, if needed, before resizing as the quality of the final product will be better.
File Compression also known as jpg optimization is compressing the file for faster downloads. The amount of compression applied does affect the final look of the image so you may need to experiment to find your "comfort spot". Large areas of solid colors, especially red, will not compress as well as details and larger photos can not be compressed as much as smaller photos. I generally use about 26% with no visible image degradation.
No compression - 44 kb
15% compression - 15 kb
25% compression - 13 kb
50% compression - 5 kb