I don't have a blog, but I read and post on several. I'm also pretty ignorant of HTML formatting. One of the blogs I follow recently put this handy formatting key right before the comments section. It shows up with every article.
HTML Formatting Tips
Italics - <em>Your text</em>
Bold - <strong>Your text</strong>
Insert image* - <img src="http://www.yourimageurl.com">
Insert link - <a href="http://www.yourlink.com">Text of your link</a>
*Please do not insert images over 400px wide and 50KB in size. Thanks.
I'd love to see more blogs do this.