Image
From Shopify Wiki
The Liquid variable image cannot be invoked on its own. It must be invoked on a product object.
{{ product.featured_image }}
{{ product.images.first }}
{{ product.images.last }}
{{ product.images[0] }}
{{ product.images[1] }}
...
{% for image in product.images %}
{{ image.src | product_img_url: 'compact' | img_tag: image.alt }}
{% endfor %}
Contents |
image.id
The image unique ID. 
image.product_id
The id of the product the image belongs to.
It is the same as product.id.
image.position
The position of the image among other images of the product.
It is the same as the value of forloop.index when you iterate through product.images.
The fist image of the product — the one featured — has its position property set to 1.
image.src
The relative path to the product image.
Example:
products/some_image.jpg
It must be used in conjunction with the product_img_url filter.
Example usage:
{% for image in product.images %}
<div class="thumbnail">
<a href="{{ image.src | product_img_url: 'original' }}" title="{{ image.alt | escape }}">
<img src="{{ image.src | product_img_url: 'thumb' }}" alt="{{ image.alt | escape }}" />
</a>
</div>
{% endfor %}
Using image by itself is equivalent to using image.src.
image.alt
The description of the image.
To edit that description, click on the 'ALT' link at the bottom of the product image thumbnail on your product details page in your shop admin.
If no description has been provided for an image in the shop admin, its alt property — image.alt — will output the product title.