Apply the placeholder behavior for desktop only

app/widgets/Post/_post_ticket.tpl

@@ -3,7 +3,6 @@
         <img class="icon thumb" src="{$post->picture->href|protectPicture}"
              srcset="{$post->picture->href} 1280w, {$post->picture->href|protectPicture} 800w"
              sizes="(min-width: 1280px), 800w"
-             alt="{$post->picture->href}"
         >
     {elseif="!$big"}
     <span class="primary icon thumb color

themes/material/css/icon.css

@@ -43,6 +43,10 @@ img.icon.thumb {
     max-width: 100%;
     object-fit: cover;
     position: relative;
+}
+
+@media screen and (min-width: 1280px) {
+    img.icon.thumb {
         min-height: 5rem;
     }
 
@@ -60,6 +64,7 @@ img.icon.thumb:after {
         font-size: 3rem;
         text-align: center;
     }
+}
 
 span.icon.primary.thumb {
     height: 100%;