Sharon Murphy on "[Plugin: All-in-One Event Calendar] Custom MySQL query?"

Did you try:

echo get_the_post_thumbnail($post_id, 'single-post-thumbnail');

Or did you try:

get_the_post_thumbnail($post_id, 'single-post-thumbnail');

They will behave differently. If you tried both, you might try this inside your function:

$post_id = $instance->get('post')->post_id;
$thumbnail_image = get_the_post_thumbnail($post_id, 'single-post-thumbnail');

Then, try calling this in your template:

echo $thumbnail_image;

If that doesn't work, you may need to try this:

echo $thumbnail_image[0];

...or possibly this:

echo '<img src="'.$thumbnail_image[0];.'">';

I'm only making educated guesses at this point, but I do hope that helps.