Figured out the answer, see below. This snippet is added to the widget class in my custom widget file.
if (get_field('upload_image', 'widget_' . $widget_id)) {
$attachment_id = get_field('upload_image', 'widget_' . $widget_id);
$size = "full";
$image = wp_get_attachment_image_src( $attachment_id, $size );
$alt = get_post_meta($attachment_id, '_wp_attachment_image_alt', true);
echo '<img alt="'.$alt.'" src="' . $image[0] . '" />';
}