jorgepinho;10989238 wrote:What content-type does the "lead.php" returns? it has to be image/jpg or some other image mime type
Only for the browser to accept the returned contents and display it. Displaying it also requires the returned data to actually be an image. If it isn't, some browsers simply disregard it alltogether, while others such as Safari, will display a "broken image" image.
However, the browser will still request the resource, since it can't know what content-type will be returned until it is returned.
However, as far as images go, browsers will cache them, which means that you need to add something like
$unique_value_per_request_and_user = '&t='.time();
which would have the browser send a get request for this image every time the page is loaded, unless two page loads from the same browser are done within the same second.
I've no idea how iframes are treated, but I'd guess it depends on what http headers are set (by you or the web server) that concerns caching.