Hi,
In core/components/phpthumbsup/model/phpthumbsup/phpthumbsup.class.php around line 360 comment out the if like below (so it works for all contexts)
I have not tested if it works in 100% scenarios, but working for me.
//if ($this->modx->context->key == 'mgr') {
$options['src'] = '/' . $options['src'];
//}
clean modx cache, browsers cache.
It happens because modPhpthumb treats image path as relative to it's own model path if it's not prepended with /:
/home/remek/www/site/core/model/phpthumb/site/assets/pics/C31645455_1.jpg