An easy way to do this would be mimic the functionality of the form for the currency sidebox (the drop-down menu).
Below is the code for the form on version 1.3.9h as it exists on my local computer.
Code:
<form name="currencies_form" action="http://127.0.0.1/zc/index.php?main_page=index" method="get"><select name="currency" onchange="this.form.submit();">
<option value="USD">US Dollar</option>
<option value="EUR">Euro</option>
<option value="GBP" selected="selected">GB Pound</option>
<option value="CAD">Canadian Dollar</option>
<option value="AUD">Australian Dollar</option>
</select>
<input type="hidden" name="main_page" value="index"></form>
The form uses the GET method to send form data to the server. This is convenient because it shows us the information we need to create links that change the currency setting.
At any rate, here are the links you'd end-up with.
USD / American Flag
http://127.0.0.1/zc/index.php?main_page=index¤cy=USD
GBP / British Flag
http://127.0.0.1/zc/index.php?main_page=index¤cy=GBP
EUR / EURO sign or list 17 flags!
http://127.0.0.1/zc/index.php?main_page=index¤cy=EUR
NB Be sure to insert your own IP/domain name and directory path in place of mine (my IP/domain is '127.0.0.1' and my directory path is 'zc'. Change those to suit your instance of Zen Cart.)
For other currencies, just substitute a the 3 letter currency code for whatever currencies you want to offer. You must make sure the currency codes are defined in the Admin console before you can use them, Admin->Localization->Currencies
The full HTML code, that is, a hyper-link'ed flag image that changes to currency setting, would look something like this:
Code:
<a href="http://127.0.0.1/zc/index.php?main_page=index¤cy=EUR><img src="EuroFlag.png" width="30" height="20"></a>
Now you just need to figure out where to put the currency flag links on your template and hard-code the links into the PHP. Personally, I'm not a big fan of hard-coding anything into ZC that cannot be changed via the admin console, but then, I'm also a fan of getting a reasonable amount of sleep!
Hope this helps.
Best wishes,
Nao