If you need to display an image in a wide range of sizes, or if the user will zoom in on them, that can be a problem. If you start out with a small (scaled down) image, information has been permanently lost. If you magnify it (zoom in), it will be fuzzy or blocky because there simply isn't the information there to fill in the new pixels. For responsive design, it may be better to offer a variety of sizes for an image and select which one based on the screen resolution (and thus, the probable final image size when zoomed in or out a small amount). If you have something like a parts diagram, where a customer may greatly magnify it to look at details, you should start with a large (detailed) picture and accept the extra bandwidth it will take to load. You also might consider an SVG format diagram, where you're sending over the same amount of data no matter the scale (not usable for images, just line drawings).
Keep in mind that a browser's image zoom capabilities are not going to produce quite as good quality of an image as would a good photo editor like Photoshop or GIMP. Substantially decreasing the size from the transmitted original (on a browser) won't look all that good, and increasing the size, as I said earlier, will result in a bad image no matter where you do it, because the necessary data just isn't there.