Image in MPDF footer won't work as a link

Question

I'm adding the following to my MPDF footer:

<a href="#bottom"><img src="arrowdownred.jpg" width="40" height = "34" />&darr;</a>

It's showing up fine, and the ↓ character works fine as a link, but the image doesn't!

The cursor changes when I mouse over it, but it doesn't work when clicked.

Anyone got any ideas?