gistfile1.txt

$curl = curl_init();
    curl_setopt($curl, CURLOPT_HTTPHEADER, array('Authorization: Bearer ABCD1234etc','Content-Type: multipart/form-data; boundary=BOUNDARY'));
	curl_setopt ($curl, CURLOPT_URL, "https://connect.squareup.com/v1/me/items/27/image");
	curl_setopt($curl, CURLOPT_CUSTOMREQUEST, "POST");
	
	
	curl_setopt($curl, CURLOPT_POSTFIELDS, ['image_data' => '@sites/default/files/styles/artists_page/public/artists/grant.png']);
	curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, FALSE);
    curl_setopt($curl, CURLOPT_SSL_VERIFYHOST, FALSE);
curl_setopt($curl, CURLOPT_VERBOSE, TRUE);
    if(!curl_exec($curl)){
        die('Error: "' . curl_error($curl) . '" - Code: ' . curl_errno($curl));
    }
    $response = curl_exec ($curl);
    curl_close ($curl);  
	
	echo $response;