When you fire photons through the double slits, one photon at a time, they cause wave interference patterns with themselves as if each photon travelled through both slits.

You're right, I forgot about this part of it.

The interference pattern only appears after firing many many photons. They're still a photon (packet of energy) when creating an interference pattern. It's the distribution and probability of its location that changes. Not the physical "shape".

Edit: to clarify further

When not being detected, it's still just one dot that appears on the sheet. As more and more are fired, the interference pattern shows to show as each photon hits the screen

It's all about the distribution possibilities.

The meme confuses two things in quantum mechanics. One is the double slit experiment, which confirms that light behaves both as a wave and a particle. That's what the meme is showing here.

However it's also throwing in Schrodinger's, which states that until you look at something it exists in all states - the classic theoretical example being the cat in a box, which is both alive and dead until you open the box. That doesn't make much sense in the real world, but when looking at quantum particles it is provably true.

Here is another meme that more accurately explains things: https://mander.xyz/post/5143468

Just to complete the set of "principles of quantum mechanics that people know of but don't fully understand", there's the Heisenberg Uncertainty Principle, which states that you can either know the position of something or its momentum (ie where it's going). The more accurately you measure one of these, the less accurate any measurement is of the other.

Edit: However there's also what /u/[email protected] said below:

The double slit experiment is about observation.

When you fire photons through the double slits, one photon at a time, they cause wave interference patterns with themselves as if each photon travelled through both slits.

Yet if you set something up to measure which slit each photon passed through, they no longer interfere with themselves, and give you the two straight lines pattern, rather than the interference pattern.

So maybe the meme was referring to this variation on the double slit experiment, rather than Schrodinger.