How can I return a green check mark icon when my email form script returns successfully [duplicate]

I have a simple php email script. This returns a simple text message saying the email has been sent successfully. I would like to add a green check mark icon next to it.

I tried including it as an img src, but it seems the output is only recognized as pure text.

$responseCompleted = "Form successfully submitted. <img src="http://www.mywebsite.be/img/vink.png"><br>";

So how can I add the image?