A few things ...
Firstly this is the support thread for the Editable Sidebox Add-in Mod rather than the currency sidebox. Nevertheless, let's quickly answer your question.
The easiest way to do what you want is to use the Blank Sidebox mod and follow the instructions embedded in that mod for inserting content such as images, and then just turn off the currency sidebox.
If you really want to hack the currency sidebox then you should replace the following:
$content .= zen_draw_form('currencies_form', zen_href_link(basename(ereg_replace('.php','', $PHP_SELF)), '', $request_type, false), 'get');
$content .= zen_draw_pull_down_menu('currency', $currencies_array, $_SESSION['currency'], 'onchange="this.form.submit();"') . $hidden_get_variables . zen_hide_session_id();
$content .= '</form>';
and lose the </html> tag that is corrupting your page's structure.
$content .= zen_image(DIR_WS_IMAGES . 'paypal.gif', 'Paypal logo', '148', '112');