Are you talking about the button image on a Button shortcode? If so, take a look at the generated script shown on the shortcode generator page (WP Dashboard → s2Member (Pro) → PayPal Buttons).
The generated script will look something like this:
<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="business" value="paypal@successfunnels.online" />
<input type="hidden" name="cmd" value="_xclick-subscriptions" />
<!-- Instant Payment Notification & Return Page Details -->
<input type="hidden" name="notify_url" value="https://clean.sitegeeks.com/?s2member_paypal_notify=1" />
<input type="hidden" name="cancel_return" value="https://clean.sitegeeks.com/" />
<input type="hidden" name="return" value="https://clean.sitegeeks.com/?s2member_paypal_return=1" />
<input type="hidden" name="rm" value="2" />
<!-- Configures Basic Checkout Fields -->
<input type="hidden" name="lc" value="" />
<input type="hidden" name="no_shipping" value="1" />
<input type="hidden" name="no_note" value="1" />
<input type="hidden" name="custom" value="clean.sitegeeks.com" />
<input type="hidden" name="currency_code" value="USD" />
<input type="hidden" name="page_style" value="paypal" />
<input type="hidden" name="charset" value="utf-8" />
<input type="hidden" name="item_name" value="Member / Description and pricing details here." />
<input type="hidden" name="item_number" value="1" />
<!-- Configures s2Member's Unique Invoice ID/Code -->
<input type="hidden" name="invoice" value="<?php echo S2MEMBER_VALUE_FOR_PP_INV(); ?>" />
<!-- Identifies/Updates An Existing User/Member (when/if applicable) -->
<input type="hidden" name="on0" value="<?php echo S2MEMBER_CURRENT_USER_VALUE_FOR_PP_ON0; ?>" />
<input type="hidden" name="os0" value="<?php echo S2MEMBER_CURRENT_USER_VALUE_FOR_PP_OS0; ?>" />
<!-- Identifies The Customer's IP Address For Tracking -->
<input type="hidden" name="on1" value="<?php echo S2MEMBER_CURRENT_USER_VALUE_FOR_PP_ON1; ?>" />
<input type="hidden" name="os1" value="<?php echo S2MEMBER_CURRENT_USER_VALUE_FOR_PP_OS1; ?>" />
<!-- Controls Modify Behavior At PayPal Checkout -->
<input type="hidden" name="modify" value="0" />
<!-- Customizes Prices, Payments & Billing Cycle -->
<!--<input type="hidden" name="amount" value="0.01" />-->
<input type="hidden" name="src" value="1" />
<input type="hidden" name="srt" value="" />
<input type="hidden" name="sra" value="1" />
<input type="hidden" name="a1" value="0.00" />
<input type="hidden" name="p1" value="0" />
<input type="hidden" name="t1" value="D" />
<input type="hidden" name="a3" value="0.01" />
<input type="hidden" name="p3" value="1" />
<input type="hidden" name="t3" value="M" />
<!-- Displays The PayPal Image Button -->
<input type="image" src="https://www.paypal.com/en_US/i/btn/btn_xpressCheckout.gif" style="width:auto; height:auto; border:0;" alt="PayPal" />
</form>
This line <input type="image" src="https://www.paypal.com/en_US/i/btn/btn_xpressCheckout.gif" style="width:auto; height:auto; border:0;" alt="PayPal" />
displays the button. You just need to change the src
to a URL pointing to the button image you want to use.