I hadn't read all your posts since my previous one, and also, I was under the impression that you were building with Materialize. Now I see you are using Foundation.
So, regarding image sizes: Nearly all my components need the 'standard' way of sizing images, therefore I haven't mentioned it in particular. By 'Standard' I mean images about as large as the maximum display size you want. It is common sense that if you need an image to display, say, 600x400 px, you take your images of maybe 6000x4000 px (mine are like that right from the camera) through Photoshop or some other photo editing software first, to give them some treatment, maybe cropping, maybe giving the colours a treat, and definitely putting them in an optimum size. In 'the old days', when bandwidth played a more important role, you probably made separate thumbnails. Many people still do that.
The bulk of my components don't need anything special for the images other than the 'standard', but if there IS a need for something special, like e.g. for the parallax components, I do mention it. Also, when using different sizes of an image for different viewport sizes.
I'm trying to explain any 'tricks' used to create the components and how to work with them. Whether my explanations are good enough, will be for others to judge about, and if something seems to be lacking, I would like to be notified, so that I can do something about it.