There are several methods that can be used to send graphics files (and other electronic files) on the Internet.

The file could be sent as an e-mail attachment.  Normally e-mail is just a text message, but it is very easy to "attach" another file to send along with the message.  If we were to draw an analogy between the Internet and traditional postal services, this is like sending a letter to another person and enclosing a photograph.  However, unlike the traditional photograph, the sender of the e-mail usually retains a copy of the file, so that when the e-mail is sent, a copy is made.  The recipient of an e-mail attachment can easily send copies of the file to all his or her friends, while retaining a copy for him or herself.  Most of the time e-mail is private because it's between the sender and the chosen recipient.  However, sometimes the e-mail is part of a mailing list with a wider distribution, even in the hundreds or thousands.

The file could be kept in a central location so that anyone who knows about the file, or has been given access, can download (retrieve from the central location) a copy for him or herself.  This is similar to having a library where any patron can check out a book, except instead of simply borrowing the book that is available, the patron takes away a copy and leaves the original book in the library, and is not obliged to return the copy to the library.  In this case, the central location could be a website or an FTP site (a place on the Internet that doesn't have to have webpages, but just files available for downloading using a communication method called File Transfer Protocol).  Sometimes, access to the files is restricted to people with the right membership or password.  Most of the time, the files are stored on a web server which is always connected to the Internet, and always available for downloads.  Sometimes, the file is made available on a single user's computer, and it can only be downloaded whenever that particular computer is connected to the Internet.  That is how Napster and similar systems work.