<button value="submit" class="submitBtn"><span>Submit</span></button>

/* REQUIRED BUTTON STYLES: */		
button { 
	position: relative;
	border: 0; 
	padding: 0;
	cursor: pointer;
	overflow: visible; /* removes extra side padding in IE */
}

button::-moz-focus-inner {
	border: none;  /* overrides extra padding in Firefox */
}

button span { 
	position: relative;
	display: block; 
	white-space: nowrap;	
}

@media screen and (-webkit-min-device-pixel-ratio:0) {
	/* Safari and Google Chrome only - fix margins */
	button span {
		margin-top: -1px;
	}
}


/* OPTIONAL BUTTON STYLES for applying custom look and feel: */		
button.submitBtn { 
	padding: 0 15px 0 0; 
	margin-right:5px; 
	font-size:2em; 
	text-align: center; 
	background: transparent url(images/btn_blue_sprite.gif) no-repeat right -140px; 
}
	
button.submitBtn span {
	padding: 13px 0 0 15px; 
	height:37px; 
	background: transparent url(images/btn_blue_sprite.gif) no-repeat left top; 
	color:#fff; 
}
	
button.submitBtn:hover, button.submitBtnHover { /* the redundant class is used to apply the hover state with a script */
	background-position: right -210px; 
}
	
button.submitBtn:hover span, button.submitBtnHover span {
	background-position: 0 -70px;
}

original source - http://www.filamentgroup.com/lab/update_styling_the_button_element_with_css_sliding_doors_now_with_image_spr/

Overides default styling

form button styling

form, css, button

CSS