Oh and being new to Wordpress does that make you new to HTML also?
A comment is something in the code but visitors on a web page cannot see it. Example should this be new to you...
<!-- <img src="http://www.csv2post.com/images/halfpricesale.jpg" alt="May half price sale now on for Wordpress 3.6 release"> -->
The <!-- is the part that makes the img become a simple comment readable only being viewing source code. Now I considered the yoast plugin checking image size. As maybe it is designed to ignore decoration.
Please see this page for further attributes and possibly improve it by adding image width/height.
