img tag inside a tag or giving background image to a tag which one is better

Considering SEO which开发者_Python百科 one is better. IMG tag inside a tag or giving background image to the a tag. Please suggest which one suits well for seo purpose.

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If image is relevant to content

For SEO - keep image in HTML and give a relevant name to image and describe the image in alt attribute.

If image is purely decorative than keep in CSS.

Edit

And read this article to understand the proper use with link

http://webaim.org/blog/alt-text-and-linked-images/