Your syntax is not following the user guide's instructions. http://ellislab.com/expressionengine/user-guide/modules/channel/grid.html#id8

You need to add the prefix for each column.

{images}
    <img src="{images:file_url}" alt="{images:caption}" title="{images:caption}" />
{/images}

Your syntax is not following the user guide's instructions. http://ellislab.com/expressionengine/user-guide/modules/channel/grid.html#template-tags

You need to add the prefix for each column.

{images}
    <img src="{images:file_url}" alt="{images:caption}" title="{images:caption}" />
{/images}