While working with images you need to establish best balance between image display quality and the file size of an image, which is also called Image optimization. You need to chose proper image format with specific levels of quality attributes to keep image size to the minimum. Optimized image loads quicker, easier to share and cause less strain on resources.
Use Save for Web in Photoshop
Generally we tend to use File > Save As – select the image format among JPG, GIF, PNG etc and save the final image. Here we can only select the image format type and save the final image. However, by using ‘Save for Web’ besides the image format, you can also customize various other aspects of image and make it more optimized.
This includes: transparency, matting, dithering, colors, quality and lots more. Just goto File > Save for Web (in photoshop), you can see maximum of 4 preview windows showing same image with different image properties. Image settings are very subjective, play around with different image settings / options and be master at image optimization.