If image dimensions are not specified in the SRC tags of an image, the server must query that image and pull its dimensions before it begins to display the image. This overhead can be saved by inputting that information into the HTML of each image on the site. This will remove one processing step on the server, and could free up server resources and increase site response time.
It also could have a visual impact on the site. When a page loads images without specified dimensions can sometimes jump around as it re-sizes itself.