Better to not change the code files. I think the CSS route is better (certain browsers are probably intelligent enough to not load the images if they are not being shown). Pls insert the following CSS into Custom CSS tab in the Theme Options panel or the /custom/custom.css file
I don’t feel comfortable with an option to disable them for blogs in options. Looks like yours is a custom requirement – not having images show up in blogs but display them in blog shortcodes. I would leave it to the user to not set the featured image. Hope you agree. Thanks.