what is image dimensions ?

Here is an example image tag without image dimensions...
<img src="image.jpg"/>

Here is an example image tag with image dimensions included (height and width)...
<img src="image.jpg" width="500" height="400"/>  

why Specify image dimensions ?

Specifying a width and height for all images allows for faster rendering by eliminating the need for unnecessary reflows and repaints.

Adding the height and width to your images can really help your webpages load faster and look better while they are loading.

Don't use width and height specifications to scale images on the fly. If an image file is actually 60 x 60 pixels, don't set the dimensions to 30 x 30 in the HTML or CSS. If the image needs to be smaller, scale it in an image editor and set its dimensions to match (see Optimize images for details.)