using the "none.png" image to represent upcoming special issues

index.html

@@ -30,7 +30,7 @@
 				<a href="18/index.html">
 					<div class="issue-nr">Special<br> Issue 18</div>
 					<div class="issue-date">April/July 2022</div>
-					<div class="issue-img"><img src="img/special-issue-18.png"></div>
+					<div class="issue-img"><img src="img/none.png"></div>
 					<div class="issue-title"></div>
 					<div class="issue-text">
                       <p></p>
@@ -42,7 +42,7 @@
 				<a href="17/index.html">
 					<div class="issue-nr">Special<br> Issue 17</div>
 					<div class="issue-date">January/April 2022</div>
-					<div class="issue-img"><img src="img/special-issue-17.gif"></div>
+					<div class="issue-img"><img src="img/none.png"></div>
 					<div class="issue-title">...</div>
 					<div class="issue-text">
                       <p>...</p>
@@ -53,7 +53,7 @@
 				<a href="16/index.html">
 					<div class="issue-nr">Special<br> Issue 17</div>
 					<div class="issue-date">September/December 2021</div>
-					<div class="issue-img"><img src="img/special-issue-16.gif"></div>
+					<div class="issue-img"><img src="img/none.png"></div>
 					<div class="issue-title">...</div>
 					<div class="issue-text">
                       <p>...</p>