If you are trying to import images from a remote server using

"image","small_image","thumbnail"
"http://example.com/img1","http://example.com/img2","http://example.com/img3"

and Image attributes processor with debug mode enabled reported:
Image attributes processor vX - error copying media/catalog/product/x/y/x.jpg; No URL set!
if the image URL displays the right image then it is probably because the php function copy is not enabled on your server, to fix this open
magmi/plugins/extra/itemprocessors/imageprocessor/imageitattributeemprocessor.php
and modify the function

public function saveImage($imgfile, $target)
    {
        $result = $this->_mdh->copy($imgfile, $target);
        return $result;
    }

by

public function saveImage($imgfile, $target)
    {
       
                $outputfile = $this->magdir ."/".$target;
		$ch = curl_init();
		curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
		curl_setopt($ch, CURLOPT_URL, $imgfile);
		curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
		curl_setopt($ch, CURLOPT_BINARYTRANSFER, 1);
		curl_setopt($ch, CURLOPT_HTTPHEADER, array('Expect:'));
		curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
		curl_setopt($ch, CURLOPT_FAILONERROR, 0); // Fail on errors
		$data = curl_exec($ch);
		$httpcode = curl_getinfo($ch);
		curl_close($ch);
		if ($httpcode['http_code'] == 200) {
		 $fh = fopen($outputfile, "w");
		 fwrite($fh, $data);
		 fclose($fh);
		 return true; 
		}
    }