Edit in JSFiddle

<form method="post">
	<input type="url" name="image" pattern="(https?:\/\/.*\.(?:png|jpeg|jpg|gif))" value = 'adres url obrazka' style="background-color:white;
                  color: #A9A9A9;"

  onblur="if (this.value == '') {this.value = 'adres url obrazka';}"
 onfocus="if (this.value == 'adres url obrazka') {this.value = '';}"
 />
	<input type="submit" value= "Wyƛlij Obrazek"/>