This trick definitely falls into the Only if you insist… category, but it'll show you a bit about how Marketo LPs work.
N.B. This post isn't about tracking email clicks in non-Marketo emails. Click tracking (via self-associating Visit Web Page events) is a very worthy endeavor and one that's the subject of another long-simmering blog post.
Rather, today's topic is an even further frontier: inserting a Marketo-aware tracking pixel that can count the number of image-enabled opens (like any tracking pixel, it only works if the lead views the HTML version of the email, has images enabled, and doesn't have a firewall that blocks such things).