Have you ever received an e-mail containing a photo so large that you can't view the whole thing on your monitor without scrolling the e-mail window? This annoyance occurs because monitors can display only a limited number of pixels. The exact number depends on the screen resolution setting, but suffice it to say that today's digital cameras produce photos with pixel counts in excess of what the monitor can handle.
Thankfully, the newest e-mail programs incorporate features that automatically shrink the photo display to a viewable size. In Windows Live Mail, for example, photos arrive with a thumbnail link to a slide show viewer that can handle even gargantuan images. That doesn't change the fact that a large photo file means longer downloading times, though — and if recipients choose to hold onto the picture, a big storage hit on their hard drives.
Sending a high-resolution photo is the thing to do if you want the recipient to be able to generate a good print. However, it's polite practice to ask people if they want to print 11 × 14 glossies of your new puppy before you send them a dozen 24 megapixel (MP) shots.
For simple onscreen viewing, I suggest limiting your photos to fewer than 1,000 pixels on the longest side of the image. That ensures that people who use an e-mail program that doesn't offer the latest photo-viewing tools can see the entire picture without scrolling the viewer window.
This size recommendation means that ...