for your block template, after you’ve registered the block with “editor”=>”attributes”=>”cover” = ‘…’
<?php if (is_admin() && @$block['example']['attributes']['cover']): ?>
<div style="background:url(<?= $block['example']['attributes']['cover'] ?>) no-repeat center #eee; background-size:contain;width:500px;height:600px;"></div>
<?php else: ?>
<!-- your block html -->
<?php endif; ?>
EDIT: eh – spoke too soon. This will change the block in the editor not just preview. Is there a conditional to determine if the block is being “previewed”?
