Your proposed approach will stop direct risks to browsers, but does nothing for ads that link to web pages that are hostile. E.g., you click on an ad because you are interested in the product and get directed to a phishing site or a site offering counterfeit goods or a site that has malware and infects your browser.
It's not just the graphic used by the ad, it's also the ad's destination.
It's not just the graphic used by the ad, it's also the ad's destination.