You could set the image as a background of the input:
background: url('image.png') no-repeat center center;
<input type="submit" value="" class="submit" />
or you could use the <button> tag:
<img src="image.png" />
The button tag is tricky, I wouldn't recommend using it if you can just use <input>
Out of curiosity, why are you using type="image"? It has a specific use.