If we are Inserting Images on the web we normally modify the size by applying the width and height attribute in the img src tag. We could make a large Image appear smaller by simply specifying the pixels for the height and width. This is well and good atleast from the display standpoint.
But one thing to note is that the Image still has the original shape wherever ever it is stored in the web server. And once the page loads, the whole Image loads and once it encounters the height and width attribute, the Image size reduces or increases. This is only visually, but in the back end the Image is actually the original size.
It would not help that we Intend to show a 100 kb Image but we always load a 1 MB file. This actually slows down the loading time for the web page. So it is always a good practice to resize the Image to your needs before adding it to the web page.
This can very well be done in Photoshop, the popular tool for editing pictures. All you have to do is open the Image in Photoshop. Goto the Image dropdown on the top, then scroll down and click Image Size. Enter the height and width of the Image you would like to have and save it.
Now Adobe Photoshop would have reduced the size of the Image to your specification.