<img{% if image.title | kebab != image.slug %} alt="{{ image.title }}"{% endif %} src="{{ image.getUrl('thumb') }}">

Example Usage:

{% if entry.images | length %}
	{% for image in entry.images %}
		<img{% if image.title | kebab != image.slug %} alt="{{ image.title }}"{% endif %} src="{{ image.getUrl('thumb') }}">
	{% endfor %}
{% endif %}