E-mail was original designed to carry “text-only” based messages. Later on, additions were made to the standards to allow the attachment of files to the messages, and the inclusion of HTML code (as used in web pages) in the message body.
There are still limitations applied though. Not everyone's software can support all the possible features, and they may just end up seeing your “fancy-featured” e-mail as a mess of gibberish. Also many ISPs place their own restrictions, usually along the lines of; limits to the sizes of individual mail that can be handled, and limits to the total amount of e-mail that can be stored at the server.
It is not good etiquette to send large e-mails, as this costs everyone time and money. Many people have a time, or byte-count, “costed” internet access, and even if the user doesn't, their ISP does (the ISP pays other networks for their access, and anything that costs them more money, will ultimately cost you too). Lots of large e-mails circulating around the system also slows everything down (ever wondered why some services suddenly slow to a crawl, and become unusable?). It's not just file attachments that make e-mails unnecessarily large. If you're replying to a message, then edit anything you're quoting—remove redundant parts of the message to bring the size down, and re-format any poorly formatted text so that it can be read easily.
In most cases, these extras are totally unneccesary, as well as being a waste. HTML enhanced (or infested) e-mail is usually at least twice the size of normal e-mail. And adding pictures to e-mails can increase the size of the e-mail tenfold. If you wish someone to see a picture you found on a website, then mail them the website address, or the individual picture's URI, instead of sending the picture; likewise, with any other files.
Bear in mind that in any case, you may be breaking copyright laws by copying files and e-mailing them to someone. Unless you are given specific permission to copy something, copyright laws protect the material's author from people copying their work, it does NOT matter that their work was made publicly available on a web site.
E-mails with attachments also carry the risk of spreading viruses. Some people will automatically delete any message (with files attached), without even looking at any of the contents, for this reason alone.