As I understand it, SEO is morphing and changing to become more and more useful - so knowing what actually impacts rankings is quite difficult and complex. There are quite a number of factors, so your best off doing a combined effort in having great Alt tags, site description tags, content, updating frequently, using descriptive links, etc. Remember Alt tags are Alternative Attribute tags and also have a function for those who are blind - so using them only for SEO purposes might mean you're making your website inaccessible.
ALT Tags hold more weight on search results than Site Description tag?

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