Images travel over the Internet much the same way as other types of email. When an email message is sent, regardless of whether it contains text or images, it’s broken up into packets of data. These data packets can be transmitted over telephone, cable, and other lines, and ultimately to the destination computer.

What happens next depends on whether you’re hooked up to a network of computers or a stand-alone PC. If your computer is part of a network, an internal routing device determines whether the email is addressed to someone on the same network as the sending computer. If the message is going to someone within a network, it’s delivered and the process is complete.

If your email is not part of a network or is going to another computer outside its own network, a device called an Internet router performs the complex process of locating and directing your message to the network the destination computer resides at.Once the Internet router directs your email to the appropriate destination network,you email might pass through both a gateway and a firewall. The gateway uses the TCP/IP protocol to reconstruct the data packets into a complete message.

A firewall is a type of security device that shields a network from the Internet so unwanted intruders can’t break in and cause damage. Occasionally, for security purposes, a firewall might be programmed so that no file attachments are allowed to pass through.For example, a network administrator might decide that image files have no legitimatebusiness purpose, and take up too much space on the company’s server.When someone sends you an email message, the message is seldom deliveredstraight to your computer. Instead, the message usually passes through a computer called an email server. The email software on your computer periodically logs into the email server and communicates with it to see if you have any mail.An Internet server might be programmed to reject email over a certain size, such as 1 megabyte, so the email server isn’t overtaxed. A megabyte is a lot of text—about a thousand pages of text like this page—but a single high-quality image might be as much as 1 megabyte.