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;

		}

    }