An image in HTML is
<img src="link to picture posted on a server somewhere" />
Problem. Not all email programs decode HTML. One that doesn't will just show the above text, not the picture. There's nothing you can do from your end to change that.
Some email programs just ignore HTML. That link will show as a blank line.
Some people, using real filtering, delete any email with HTML. I don't talk to people who send me HTML in the body (if they're sending me something for me to look at the HTML, they'll attach it in a file), so I know it's spam, so it gets deleted before I even see it. POPFile will do that after a month or so of having anything with HTML in the body set as spam.
Rule of thumb - don't put anything in an email but letters, numbers and punctuation. Every email program will show your email the way you sent it.
Pretty pictures are nice - save them for your print ads and websites.