Content disposition is one of the attributes of an email attachment defined by standards. It tells the email software to display a particular attachment either in place (inline content disposition) or as icon (attachment content disposition).
The standard Mac OS X Mail application does not respect the content disposition of attachments, nor does it set it correctly for your outgoing mail.
Attachment Tamer can ensure that Mail respects the content disposition in incoming mail and attachments in outgoing mail have correctly set content disposition, so that the recipient will see them in the way you choose. Additionally, it inserts attachments correctly in HTML layouts. If you are composing a message with images that are to be displayed in place, using rich formatting (and so embedding the images in an HTML layout) is the best way to ensure the message will be displayed consistently across different software platforms. Read more about rich (HTML) messages.
You can fine-tune how Mail interacts with the content disposition of attachments in Attachment Tamer preferences. When composing a message, content disposition of an individual attachment can be changed by selecting the option View in Place or View as Icon from the attachment context menu (you can open the menu by control-clicking on the attachment).