I think what euforico means is that he has the custom field set up for the src of an image for example.
<img src="<?php echo get_post_meta($post->ID, 'my_name', true); ?>" alt="Alt Text" />
I know what I want but my php code is very inadequate.
You need something like:
if the custom field is not empty, display this:
<img src="<?php echo get_post_meta($post->ID, 'my_name', true); ?>" alt="Alt Text" />
if it is empty, display this:
<img src="https://www.blog.com/path-to-generic-image.jpg" alt="Alt Text" />