Why HTML in E-Mail is a Bad Idea for more information. HTML can include any number of scripts, dangerous links, controls, etc.
When viewing embedded images in a recieved message that was composed in HTML can reveal your email address is valid if the message was sent by a spammer and cause other potential problems for Windows users such as security issues and trojan horses - it's a gateway to danger as any Outlook user can tell you. HTML composition with email messages may be a standard feature with Windows but it is also caused problems for Windows users and the internet as a whole and IMO, one of the biggest of many mistakes Microsoft brought to the internet. Depending on the recipient's email client and available preference settings, such an attachment may appear inline or viewed in place when the message is opened or as a separate attached file which must be opened separately. You can attach such a gif image to a message (which can be found on the internet) and although it will appear inline or viewed in place within the body of the message, it will not be embedded and will be sent as an attachment to the message.
The Mail.app does not support HTML composition - either RTF or Plain Text so you cannot embed emoticons, etc.
