In my own experiences, image optimization would regard to alt attribute, contextual, page title and image filename keyword oriented optimization.
Have a nice day,
alt attribute, which isn't a tag, there is no such tag called <alt>. and yes, it is essential for the crawler, because when blank or not placed, the image will most likely NOT be indexed in the Goolge Images archive.
<img src="bill.jpg" alt="Bill Smith during concert" />
<img src="bill.jpg" alt="" /> (W3C Valid markup)
<img src="bill.jpg" /> (W3C Invalid markup)
According to W3C recommendations, each image MUST have an alt attribute even if it's blank (alt="").