Fix archive img attributes and markup

[nickcernis]

Small corrections to archive images as detailed in https://github.com/studiopress/genesis-portfolio-pro/issues/25.

• Use genesis_markup instead of printf to make markup filterable.
• Add aria-hidden to image link to prevent the link being read twice by screen readers (the item titles are also wrapped in links).
• Add 'itemprop' => 'image', to images so they appear as part of structured data.
• Standards corrections to array formatting.