Stated image dimesions is incorrect

Your images should be scaled to required size and optimized for the web BEFORE they are uploaded to your server.

OPTION 1: USE RESPONSIVE IMAGES WITH <PICTURE> TAG

Photoshop's ExportAs feature is a good tool for this.  It permits you to create 1x, 2x and 3x sized images for various devices.  Combined with <picture> tag and srcset attribute, you can deliver the ideal sized image to appropriate devices, thus reducing excess bandwidth to mobile users.

 <picture>
  <source media="(min-width: 650px)" srcset="big_image.jpg">
  <source media="(min-width: 465px)" srcset="med_image.jpg">
  <img src="smallest_image.jpg">
</picture> 

OPTION 2: USE CSS PERCENTAGE WIDTHS

• img {max-width:100%}  downscales images to fit smaller devices but maintains native file size on large devices.
• img {width: 100%} downscales & upscales images to fit any device however distortion & pixelation may occur. 
• Remove HTML height and width values from code.

 Hope that helps.

It does not matter that the images are larger than the size they will appear on the website. However good practice is to deliver them close to the largest size that they need to appear on your website and certainly they shouldn't be so large as to cause issue like  being slow to download. A better practice is to use srcset to deliver images optimised for the device that they will be viewed on, you don't really want a 1500px wide image to be delivered on a 600px wide screen, that's were srcset can deliver the size of image applicable for the width of the screen.

Also adding px width and height directly to the img tag itself will make the image non responsive so l don't advise doing so unless you are developing a static width website for desktop. Infact using px to declare width is generally bad practice in most cases. You should be using percentage width or another flexible unit like em as they respond to a variety of screen width sizes.