Hi,
You could use the thumbnail_filler
parameter to display a placeholder when thumbnails have not been set, check out this example: Thumbnails with Placeholders
Nonetheless, you can’t display personalized placeholders, right now your options are: placeholder (by default), kittens and puppies.
Of course if you want your posts to display a thumbnail you have to set a featured image when you’re editing a post.
Finally if you want to edit the code to add your own placeholder provider, you could find and replace these lines:
// Placeholder provided by Placehold.it
case 'placeholder':
echo "<a href='".$all_permalinks[$field->guid]."'><img src='https://placehold.it/".$thumbnail_wh."&text=".$field->post_title."' alt='".$field->post_title."' title='".$field->post_title."' /></a>";
break;
// Just for fun Kittens thanks to PlaceKitten
case 'kittens':
echo "<a href='".$all_permalinks[$field->guid]."'><img src='https://placekitten.com/".$thumbnail_size[0]."/".$thumbnail_size[1]."' alt='".$field->post_title."' title='".$field->post_title."' /></a>";
break;
// More fun Puppies thanks to PlaceDog
case 'puppies':
echo "<a href='".$all_permalinks[$field->guid]."'><img src='https://placedog.com/".$thumbnail_size[0]."/".$thumbnail_size[1]."' alt='".$field->post_title."' title='".$field->post_title."' /></a>";
break;
Just remember if you make changes to the code they will be erased when updating to a new version.
Cheers.