The message would contain the HTML to display the image.
like
<img src="/images/en_US_whatever.jpg">
You can embed any HTML In the message. You can refer to variables available in the page as well by surrounding the variable name with % ie: %varname%
In this case it might not be a big win to use acs-lang but instead of your complex switch statement I would name the images with the locale so you can do something like
img src="@mailto:user_locale@_image_name.jpg";
or place them in a folder by locale
img src="/images/@mailto:user_locale@/image_name.jpg";
This is more flexible. It would be interesting to discuss other ways to abstract this type of user interface element. I still think its probably best to use the message keys, then all your localization is done in one place.