WP deleting link url in link code

I have been listing my friend’s site on toplists for years now and everything, up until this last WP update, has been fine. Now I am having some issues. The problem comes with a certain type of linking code. The toplist will give you a copy and paste link code for your website/wordpress. Here’s an example:
<a href="http://www.blahblahblah.net/"><img src="http://www.blahblahblah.net/button.php?u=usernamehere" alt="Blah Blah Blah TOPSITES" border="0" /></a>

The problem is that when this code is placed into the text section of a wordpress page it’s fine until you save/update the page. Upon updating, the banner image url has been removed though the rest of the code remains intact. It does this with every button.php style of link I’ve got.

Here’s what it will look like after an update
<a href="http://www.blahblahblah.net/"><img src="" alt="Blah Blah Blah TOPSITES" border="0" /></a>

I have tried changing themes and disabling plug-ins to no avail. I have checked that the image url works properly in a browser, and I have tried various browsers to make sure Chrome wasn’t the problem. There have been zero changes to the site with the exception of the WP update.

Does anybody have ideas on why this has started happening or know of any plugins to solve it? Please don’t assume I know too much as I am definitely not a coder, more of a cut and paster.

Thanks in advance!