Specify image dimensions in wordpress

When i ran performance report for my website (using GTmetrix & PageSpeed Insights ) and I realized the image dimensions are not set. The theme already had some images, but they are not specified by default. How can I specify them and more importantly, where do I specify the attributes height and width?

Awesome Image SHIP SPARE PARTS DECK ENGINE CABIN STORES WASTE-Sludge-Slop-DISPOSAL-FROM-SHIP MISCELLANEOUS STORES FOR VESSEL On-Request-Delivery