If you want to add an icon to a default submit button with css,
there is a solution but it seems that it works only on mozilla :
PHP Code:
<style>
.icon {
background: transparent url('/path/to/icon.png') no-repeat 3px center;
padding-left: 18px;
}
</style>
<input type="submit" class="icon" value="Click to login" />
(good with a 16x16 icon, else change the padding-left)
On other browsers, the style is ignored