The HTML "img" element has a couple optional attributes called "width" and "height". These have two purposes.
Firstly they can be used to override the size of an image and force a browser to scale it. But that's not what this page is about.
Their second use is to tell the browser what size the image is. If you don't do this then some browsers will shift things around on the page as they discover the size of the the embedded images. This means that the page jumps around as it loads. This is visually disturbing, and also means that if you try to click on something in the page while it's still loading there's a risk that it might move out of the way at the last moment.
Therefore, please use correct "width" and "height" attributes on embedded images in web pages: doing this will improve the experience of your readers.
Graphics editors and graphical web browsers will often be able to tell you how big an image is if you load it into them.
ImageMagick contains a tool called identify which will report the size of many kinds of image. It runs under UNIX, Windows, Mac OS and other platforms.
The file program found on some UNIX systems can report the size of some kinds of image.
HTML 4.01 definition of WIDTH and HEIGHT