Alt Tags are import to help navigation, and should be built with publications on your site. There are many great resources within Wealthy Affiliate to help build your alt tags. According to this article on Attorney At Law Magazine, 6 Tips to Avoid Wrong Side of a Web Accessibility Suit, A high percentage of attorney websites are not in compliance. This is risking fines for violating the equal accessibility requirements for people with visual or hearing disabilities.
The importance of understanding alt tags has been highlighted for website ranking and compliance with the necessary elements to ensure accessibility for people. This increases the fluidity of your site for more readers, and thus builds more trust and fans of your brand. Trust can grow by anticipating needs of the diversity of your website visitors. All of whom may not be able to see or hear.
Alt tags will help build your site trust and can diversify the accessibility for your site. If it sounds like the alt tag police will fine you, please humor my soap box. It is possible to lose visitors permanently if they cannot access portions of your website, such as descriptions of pictures or closed caption capabilities for videos. Even without the threat of fines, the possibility of losing visitors who require assistive devices and techonology should motivate you for building your Alt Tags.
If I recall the alt text must be about 125 characters or less and in these characters you should leave out things like "photo of" or "image of". That should be obvious. It's a good idea to use a keyword or two but no keyword stuffing, that'll kill it. Done correctly it can be a great benefit to SEO.
I don't have a specific resource but I've used moz.com for info.
Cheers,
Mike