How to add image field to paragraph programmatically?
You can treat the field like a PHP array to add (and not replace) an image:

use Drupal\paragraphs\Entity\Paragraph;

  $paragraph = Paragraph::load($paragraph_id);
  $paragraph-&gt;field_image[] = [
    'target_id' =&gt; $fid,
    'alt' =&gt; $alt,
    'title' =&gt; $title,
  ];
  $paragraph-&gt;save();