Top bar ConvertKit form

ck_form.css

.ck_form_bar {
	font-family: 'Helvetica Neue', Helvetica, Arial, Verdana, sans-serif;
	line-height: 1.5em;
	overflow: hidden;
	color: #666;
	font-size: 16px;
	border-bottom: solid 1px #d1d1d1;
	clear: both;
	margin: 20px 0px;
	padding: 20px;
	background: #f9f9f9;
	width: 100%;
	position: relative;
	position: fixed;
	top: 0px;
	margin: 0px;
	left: 0px;
	z-index: 500;
}

.ck_form_bar, .ck_form_bar * {
	-webkit-box-sizing: border-box;
	-moz-box-sizing: border-box;
	box-sizing: border-box;
}

.ck_form_bar .ck_form_bar_title {
	font-size: 30px;
	color: #333;
	text-align: left;
	margin: 18px 0px;
	font-weight: normal;
}

.ck_form_bar_content {
	width: 50%;
	float: left;
}

.ck_form_bar_fields {
	width: 50%;
	float: left;
}


/* Form fields */

.ck_errorArea {
	display: none; /* temporary */
}

#ck_success_msg {
	padding: 10px 10px 0px;
	border: solid 1px #ddd;
	background: #eee;
	text-align: center;
}

.ck_form_bar input[type="text"], .ck_form_bar input[type="email"] {
	font-size: 14px;
	padding: 10px 8px;
	width: 100%;
	border: 1px solid #d6d6d6; /* stroke */
	background-color: #fff; /* layer fill content */
	margin-bottom: 5px;
	height: auto;
	width: 58%;
	display: inline-block;
}

.ck_form_bar input[type="text"]:focus, .ck_form_bar input[type="email"]:focus {
	outline: none;
	border-color: #aaa;
} 

.ck_checkbox input.optIn {
  margin-left: -20px;
  margin-top: 0;
}
.ck_form_bar .ck_opt_in_prompt {
  margin-left: 4px;
}
.ck_form_bar .ck_opt_in_prompt p {
  display: inline;
}

.ck_subscribe_form {
	margin:  0px;
}

.ck_form_bar .ck_subscribe_button {
    color: #fff;
    margin: 10px 0px 0px;
    padding:  10px 0px;
    font-size: 16px;
    background: #0d6db8;
    cursor: pointer;
    border: none;
    text-shadow: none;
    display: inline-block;
    width: auto;
    width: 38%;
    padding-left: 2%;
    padding-right: 2%;
  }

/* Trigger the vertical layout with media queries as well */

@media all and (max-width: 1023px) {
	.ck_form_bar_content, .ck_form_bar_fields {
		width: 100%;
		float: none;
	}
}

@media all and (max-width: 611px) {
	.ck_form_bar input[type="email"] {
		width:  100%;
	}
	.ck_form_bar .ck_subscribe_button {
	    width: 100%;
	}
}

ck_form.html

<script src="https://app.convertkit.com/assets/CKJS4.js?v=18"></script>

<div class="ck_form_bar">
	<div class="ck_form_bar_content">
		<h3 class="ck_form_bar_title">Join the newsletter</h3> 
	</div>
	<div class="ck_form_bar_fields">
		<div id='ck_success_msg'  style='display:none;'>
			<p>Check your email to download the free resources.</p>
		</div>
		<!--  Form starts here  -->
		<form id="ck_subscribe_form" class="ck_subscribe_form" action="https://app.convertkit.com/landing_pages/000/subscribe" data-remote="true">
			<input type="hidden" name="id" value="000" id="landing_page_id"></input>
			<p class="ck_errorArea"></p>
			<input type="email" name="email" class="ck_email_address" id="ck_emailField" placeholder="Email address" required></input>
			<button class="subscribe_button ck_subscribe_button btn fields" id='ck_subscribe_button'>Join the list</button>
		</form>
	</div>
</div> 

Hey Nathan - One of your CK newbie customers here.
I'm loving Convertkit, and am trying to work out how to have an image as a button. Can I save a button image in wordpress as media and do some basic coding, or can I use the button image saved in wordpress?? Can't figure out the change.
Thanks Belle