In the first place know that email clients have the worst support for HTML and the most inconsistent support from one client to the next. You are not alone, getting email to work is a frustrating experience and it takes a lot of work to achieve what seems like relatively little results. So this is normal, everyone has a hard time with HTML for email.
On your image and rendering issues, it sounds like those emails may be rendered in text. The image issue could be complicated, there's a lot that can go wrong with images in email. However, when your HTML is being treated as content, that tells me that while you're *sending* the email as HTML, the account that *receiving* the email is treating it as text. This could be because of a setting in the receiving account, or because your account's security is concerned about something in the email (are there any scripts? An image "might" do it, but that seems like a pretty restrictive configuration). I would get the email rendering as HTML first, because the image wouldn't show up if it's just treating the <img> as content. If it will render the HTML and STILL doesn't display the image, then start looking into all the other reasons the image could be a problem.
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