If I am not mistaken, when you add any code to an like add an image, or a internet address etc, the e-mail becomes html active.
I use copy/paste rather than an attachment. The image is viewable before you and after the mailing. No need to upload it to a site.
Here are ways that you can use if you don't have an internet address.
You might be able to make a web page and copy-paste it to the e-mail, but you will have to copy-paste the images in the right places later so they can be sent with the e-mail.
You can also make a page for instance in Microsoft Publisher, and mail it from within. Or, use what they call "Stationery", do a search for information on it.