Add styling for post images

We don't want them to fall back to default behaviour, because that
involves being too large and spilling out of the screen!

static/style.css

@@ -111,6 +111,21 @@ h2 + h3 {
   padding: 1em;
 }
 
+./*
+  * Styling for images. We centre-align them, make sure
+  * they fit in the screen, and see that they don't
+  * spill out of the post container.
+  */
+.main p:has(img) {
+  text-align: center;
+}
+
+.main img {
+  max-width: min(40em, 100vw);
+  max-height: 100vh;
+  margin: 1em auto;
+}
+
 .post-list {
   padding: 0;
   list-style-type: none;