Output a default asset if one hasn't been specified
You can test if your first() method returns an asset model. 
And depending on that have different image URLs, the (transformed) asset's one or the URL of your placholder image.

{% set image = block.mx_peoplePhoto.first() %}
{% set imageUrl = image ? image.url('500x500') : url('images/_static/placeholder.jpg') %}

&lt;img src="{{ imageUrl }}"&gt;


You could also consider to get that placeholder image from a global set.