Here is an example of a photo being uploaded to a website and then using the handlebars in the editor to show the large image at a smaller size:
Website visitors still have to download the larger image even though it's being shown at smaller dimensions.
See how it comes in slowly? This is the wrong way of resizing images.
You should resize them in image editing software before uploading it to your website.
Right click the image and click "View Image" (in Firefox) to view the actual image.
Size: 5.3 MB
Shown Dimensions: 500 x 333
Actual Dimensions: 3888 x 2592