This technique is used quite a lot to get an image styled url of an image in your twig file.
use Drupal\image\Entity\ImageStyle; // on top
function THEMENAME_preprocess_node(&$vars) {
if(($node->getType() == 'partner_logo') && ($variables['view_mode'] == 'teaser')) {
$id = $node->field_logo->entity->getFileUri();
$url = ImageStyle::load('partner')->buildUrl($id);
$variables['image_url'] = $url;
}
}
Then, in your partner-logo--teaser.html.twig:
<div style="background-image:url('{{ image_url }}'); background-size: cover"></div>