Macro to prevent upscaling - something Craft doesn't support just yet...

index.twig

{# Usage #}
{% set transform = { width: 700, height: 247, mode: 'crop' } %}
<img class="banner" src="{{ _self.transform(entry.bannerImage.first, transform) }}">

{# Macro to prevent upscaling - something Craft doesn't support just yet... #}
{% macro transform(image, transform) %}
{% spaceless %}
    {% set originalWidth = image.getWidth(false) %}
    {% set transformedWidth = image.getWidth(transform) %}

    {% set originalHeight = image.getHeight(false) %}
    {% set transformedHeight = image.getHeight(transform) %}

    {% if transformedWidth > originalWidth %}
        {% set transform = transform | merge({ width: originalWidth }) %}
    {% endif %}

    {% if transformedHeight > originalHeight %}
        {% set transform = transform | merge({ height: originalHeight }) %}
    {% endif %}

    {{ image.url(transform) }}
{% endspaceless %}
{% endmacro %}