Image file size - how much space it takes to store the image - is measured in kilobytes (KB), or megabytes (MB) if the image is very large. This is determined by the height and width of the image and by the resolution. Modifying either or both of these can result in an image that takes much less storage space and will be more convenient to email or will load more quickly on a website.
...
Height and width can be measured in a number of different units depending on your preference - inches, centimeters, didots, pica points, and pixels.
...
Resolution is measured in DPI - Dots Per Inch - and should never be more than 96 dpi - with 72 dpi being the preferred selection. Most digital cameras will default to 72 dpi resolution.
...
Another factor that affects the file size of a picture is the format in which it's saved. Image formats, such as .tif and .bmp, are made for the highest quality, are not compressed, and will produce larger image files.
...
Image formats such as .gif and .jpg, were designed for fast loading. For example, a photo saved in .tif format that is 850KB in size will only be 255KB when saved as a .jpg file.  Most digital cameras will default to .jpg format.  Additionally, if you plan to use your digital photo on a website - it must be in either .gif or .jpg format.

Why does image file size matter in an email?

To keep their servers from being overloaded, most email services will only allow messages of up to a certain size, like 2MB or 5MB. If you send a message that's larger than the upper limit, then the email will be kicked back with an error message. Or, if it does squeak through, it can open in your recipient's email client as a very long mess of confusing alphanumeric lines. An oversized email can also fill up your recipient's mailbox, causing other incoming emails to be rejected.
...
Another consideration is the time your recipient must spend waiting for the email with your large image file to download. If you send a huge 6MB image to someone who uses a dial-up connection to the Internet, it can take them an hour or more to receive your message and your recipient probably won't be happy about that.

What is considered an acceptable digital photo file size?

Since most monitors display 800 x 600 pixels, you should never exceed that size. But is it really necessary to take the entire screen for you photo? A good rule of thumb is to resize the image so the largest side is no larger than 640 pixels. Experiment by resizing some photos and emailing them to yourself. You may find that resizing your photo so the largest side is just 400 pixels still produces a very good image.
....
An original digital photo that measures 2048 pixels by 1536 pixels and takes 734KB storage space can be resized to 640 pixels by 480 pixels and will only take 33KB storage. That's less then 5% of the original file size!

Resizing is super easy with Windows XP

If you have Microsoft Windows XP resizing digital photos is simple...

• Open the folder with your images and select the one you want to send. (hold down Ctrl while clicking to make multiple selections.)
• Next, in the Task Menu on the left side of the screen, click "E-mail this file" (or "E-mail the selected items" if you've selected several images) from within the "File and Folder Tasks" section.
• You'll see a dialog box that asks if you want to make the picture smaller or keep the original size.
• Select "Make all my pictures smaller". The original image(s) will not be affected.
• Your email program will open with the smaller image(s) already attached

Resize using the FREE IrfanView program

IrfanView is simple freeware image viewer and editor that supports all major graphic formats, including BMP, DIB, JPEG, GIF, animated GIF, PNG, PCX, multipage TIFF, TGA, and more. It's free, it's fast and it uses very little system resources. In addition, it features drag-and-drop support, directory viewing, TWAIN support, slide shows, batch conversion (so you can resize lots of photos at once), and modifications such as color depth, crop, blur, and sharpen.
...
Here are three easy ways to resize your digital photo with IrfanView..
...
Resize by half...

• Select... File, Open, Pictures (folder), select the image you want to resize and click Open.
• Select... Image, Resize/resample, click the Half button to resize the picture. Click OK.
• Save the resized image.

Resize by pixels...

• Select... File, Open, Pictures (folder), select the image you want to resize and click Open.
• Select... Image, Resize/resample and note the current image width and height size in pixels
• Resize your image by typing a new Width or Height under the New size option. Click OK.
• Save the resized image.

Resize by percentage...

• Select... File, Open, Pictures (folder), select the image you want to resize and click Open.
• Select... Image, Resize/resample, Percentage of original and type in the desired percentage you want to reduce your image by.
• Save the resized image.