Looks like that space is being created by the break tag <br> here:
<p>
<input type="hidden" name="currency_code" value="USD">
<br>
<input type="image" alt="PayPal - The safer, easier way to pay online!" name="submit" src="http://slowfoodportland.com/buy-ticket.gif">It's not visible in your HTML code above (but that may have been messed up by posting here without using the code buttons) - are you using the TEXT editor to add the form? The VISUAL editor may be adding or messing up code if you're using it or switching between them.