For future reference just attach the image to your thread instead of uploading a document with it inside of it (no one likes to download unknown files!).
Again, I have no idea how you're trying to render the button; you just use the HTML that PayPal gives you. It does not look like you're using HTML, but instead using some sort of Joomla content bot, which CB does not process. Example as follows.
Code:
<a href="DONATE_BUTTON_EMAIL_CODE" target="_blank"><img border="0" src="https://www.paypalobjects.com/WEBSCR-640-20110429-1/en_US/i/btn/btn_donate_LG.gif" alt="PayPal - The safer, easier way to pay online!" /></a>
Note: You can not use PayPals "form" based button as registration is a form itself and you can't have a form inside of a form. When you generate a button it has 2 codes, form and email, and you'll want to use the email code in the above HTML. If the image isn't to your liking the take the image input value from the form code.