[css-align] Add issues to the hand-drawn images tomake it clear these are placeholders until we draw real pictures.

Author: tabatkins

css-align-3/Overview.bs

@@ -1108,6 +1108,9 @@ Overflow and Scroll Positions</h3>
 
 	<figure>
 		<img src="images/scroll-align-padding.jpg" alt="">
+		<figcaption>
+			Issue: Replace this image with a proper SVG.
+		</figcaption>
 	</figure>
 
 	ISSUE: Coordinate wording with css-overflow once it's less of a mess.
@@ -1123,10 +1126,15 @@ Overflow and Scroll Positions</h3>
 	grows the <a>scrollable overflow region</a> but not the <a>alignment subject</a>,
 	thus an ''align-content/end''-aligned <a>scroll container</a>
 	might not be initially scrolled all the way to the bottom.
+
 	<figure>
 		<img src="images/scroll-align-overflow.jpg" alt="">
-		<figcaption>Overflow is not part of the <a>alignment subject</a>,
-		even for a <a>scroll container</a>.</figcaption>
+		<figcaption>
+			Overflow is not part of the <a>alignment subject</a>,
+			even for a <a>scroll container</a>.
+
+			Issue: Replace this image too.
+		</figcaption>
 	</figure>