Where did you place “my-class” on the module? Under ID or Class? From your CSS, since you are using .my-class, you should place it under Class. If you place it under ID, then you should change your CSS to #my-class.
I’m gonna need to have a look at the site you are working on for me to check thoroughly. Do you think you could send me the site URL? If you could grant temporary admin access as well that would be great.
I checked the page and the CSS seems to be applied to the image? Furthermore, your CSS is on top of the list so it should override any default CSS applied to the img element. Check screenshot. http://imgur.com/phMpyKo