How i can set default image if image not found

if image not uploaded form admin in this case i want to set default image

{% set first_image = ps.media.images|first %}
{{ first_image.cropZoom(900,300).html() }}

Assumed default.jpg is in the user/pages/images folder, you can do something like this:

{% set firstimage = ps.media.images|first %}
{% set defaultimage = page.find('/images').media['default.jpg'] %}

{% if firstimage %}
  {{ firstimage.cropZoom(900,300).html() }}
{% else %}
    {{ defaultimage.cropZoom(900,300).html() }}
{% endif %}