Thanks for @dunc and @helgatheviking I got the answer. Here’s my code. You need the post type declared, otherwise it resorts to “post”. I also couldn’t do this unless I put meta_key and meta_value in a ‘meta_query’ multidimensional array.
$args = array(
'post_type' => 'tsa_events',
'meta_query' => array(
array(
'key' => 'slideshow_image',
'value' => array(''),
'compare' => 'NOT IN'
)
)
);
EDIT: you can also structure your query this way:
$args = array(
'post_type' => 'tsa_events',
'meta_key' => 'slideshow_image',
'meta_value' => array(''),
'meta_compare' => 'NOT IN'
);