Via Shaun Anderson | @Hobo_Web
Text in Alt tags are 'counted' by Google (and Bing) on a page level, but be careful over emphasizing them – as with every element of SEO in 2016. A lot of websites penalized for over-emphasizing invisible elements on a page, so don't do it. However, ALT tags are significant and an important challenge to get right.
Use ALT attribute text for descriptive text that helps visitors – and keep them unique where possible, like you must do with your titles and meta descriptions. Variation = traffic. And at the very least, use a blank ALT (or NULL value) so people with screen readers can enjoy your page without unnecessary interruption, too.