When a web page is being downloaded to a computer, it is receiving multiple files at once, this includes the html page as well as all the images. By the time the images have been transmitted the HTML file has long since finished, yet unless the browser knows in advance how much page space to leave for these images, it has to wait until every image has at least begun downloading in order to complete the text layout of the HTML file. These are used to tell the browser how much space to leave for the images so the HTML page can be downloaded around the images allowing your reader much earlier access to the material. Most HTML editors will have a function that you can turn on to do this automatically when you insert images but you can find out the height and width of image by opening it up in an Image Editor and clicking on image size. The commands are denoting the image's height and width in pixels.
Pixels are little colored dots.
If you put enough of them close together, they form a picture. You can alter the images shape also by putting in different height and width commands. If the image is really 50X50 and you put in height and width commands of 150 and 10, you will distort the picture pretty well. The code to use to specify the size of the image is as follows. The compression can be adjusted so that less compression can be used when higher quality is needed but more compression can be used when quality isn't as important.