Using an image for site title

Possibly, if you use appropriate alternate text for the image. Better yet, set the image as a background to a header tag and “hide” the header text offscreen using text-indent. Then you get the SEO benefit of structured markup:

<h1 class="site-title">My Awesome Website</h1>

.site-title {
   background-image:url('http://url.to/backgroundimage.jpg');
   background-repeat: no-repeat;
   text-indent: -9000px;
   width: 500px;
   height: 250px;
}

(Obviously replacing width/height with the width and height of your background image.)