.bilder_1 

img:hover {
transform: scale(1.8);
z-index: 100;
border: 3px white solid;
}
/*
<div id="bilder_1">
  <div id="content">
  <h1>Projekte</h1>
    <img src="Bilder/Plakat_Slow_Food 3.png" align="left" height="200" alt="Plakat Slowfood"/>
   <img src="Bilder/Werbung.jpg" align="left" height="150" alt="Werbung"/>
  ....
   <img src="Bilder/feld.jpg" align="left" height="150" alt="Landschaftsfoto"/>
   <img src="Bilder/zapfen.jpg" align="left" height="150" alt="Tannenzapfen"/>
  </div>
*/

.bilder_2 

img:hover {
transform: scale(2.8);
z-index: 100;
border: 3px white ridge;
}