Use image scale and crop in custom function
You problem is image_load($img_url) is returning FALSE, which is not an object.   

$filename = $bjt_img;
$img_url = "http://www.mydomain.com/uploads/".$filename;
if (isset($myimg = image_load($img_url)) {
  $myimg_scaled = image_scale_and_crop($myimg, 100, 80);
  print '&lt;img src="'.$myimg_scaled.'"&gt;'
} 
else {
  print 'error'
}


Plus I think it must be a file,  not a url to a file