I've written before about sizing images, but because so many people are just now getting digital cameras, I still get a lot of questions on the subject.
Just in the last week, my father and a client told me that they haven't yet figured out how to make their digital photos smaller, so they can e-mail them to people. As both discovered, sending a 3-megabyte file to your cohorts who use dial up connections does not win friends. The recipient either won't download the photos, or if they do, you'll really hear about it. (And not in a good way.)
Big files take a long time to download. The reason relates to a concept called "resolution."
Resolution is a way to measure the number of dots (called pixels) in an image. The higher the resolution, the more pixels and the larger the file. So suppose you have two images that are the same size (height and width). One image is 400 dpi (dots-per-inch), and the other is 200 dpi. The 400 dpi image has four times as much detail as a 200-dpi image because it has four times as many pixels (200 x 200 = 40,000 vs. 400 x 400 = 160,000). When you lower the resolution, you lose detail, but the file size decreases. When working with images, you always have a trade off between image quality and file size.
The dimensions of the image also affect the number of pixels. For example, suppose your image is 1024 pixels wide by 768 pixels high. That's obviously going to be larger than an image of the same resolution that is only 512 pixels wide x 384 pixels high. Again, more pixels mean more data, which means a larger file size.
So with that in mind, you can probably guess that today's digital cameras take high-resolution pictures. Before you e-mail your image file, you need to reduce both the resolution and the size of the image. Fortunately, in most image editing software, the process is easy.
For e-mailing, you want to set the resolution to 72 dpi. You can set the height and width of the image to more or less anything you want, but again smaller is better. When I e-mail photos, I generally go for something less than 400 pixels wide. If your software only shows you dimensions in inches, don't go any larger than 4x6.
Often you can choose the file format as well. For e-mailing digital photos, select .jpg, which is a compressed file format. You want your picture to be as small as possible, so compression is good.