The "Alternative Attributes of an Image Tag" or the ALT tag as it is more commonly known was developed to improve accessibility for impaired visitors to your website.
Over the last few years Alt tags have somehow become confused with SEO elements, this is not true and can actually hinder your SEO if the tag itself is optimized for SEO in a way thus taking away from the true meaning of the tag by giving a description of the image element.
The needs of SEO are best served by keeping the true meaning of the attribute in the forefront, accessibility for the impaired.
Write short, descriptive alt attributes for images that assist your users in understanding what the web page is about and how to perform their actions. In doing so, you’ll also be optimizing for SEO without having to worry about stuffing too much content or too many keywords into each one.