<img class="triangle" src="image.png"/>
CSS
.triangle{
??}
Now I want to apply css to the triangle class to show the image in a triangle. 
<img class="triangle" src="image.png"/>
CSS
.triangle{
??}
Now I want to apply css to the triangle class to show the image in a triangle. 
 
    
     
    
    I've made you a fiddle here. You need to wrapp your image with a div to do what you want:
.triangle{
    position: relative;
}
.triangle:before{
    border-top: 0;
    border-right: 190px solid white;
    border-bottom: 240px solid transparent;
    border-left: 170px solid white;
    position: absolute;
    content: "";
}<div class='triangle'>
    <img src="http://s.hswstatic.com/gif/landscape-photography-1.jpg"/>
</div>