Reading up on the James Webb telescope and how the images from it show us the universe as it was 13.5 billion years ago has made me wonder what does this mean? Are we in essence viewing the past? And if the universe is always expanding does that mean there is more behind these images?
Yes. Exactly that. When we look at anything we see it in the past. When I hold my hand 30cm in front of my face, I see how it was one nanosecond ago. When you see the moon, you see it 1.2 seconds ago. When you see Sirius you see how it was 8 years ago. And when the JWST sees a very distant galaxy, it sees how it was 13.5 billion years ago.
As far as we know, the universe is unbounded in extent. it is much larger than the observable universe. So there would be more space behind it now. However we can't see much more behind it. While the universe seems to be unbounded in space, it isn't unbounded in time. The universe began 13.8 billion years ago, and so there is nothing to see that is older than that.
Are we in essence viewing the past?
That is the essence of a telescope. Telescopes are time machines that see into the past. Electromagnetic radiation (which includes visible light) travels at the speed of light. When one views even the closest star (other than the Sun), what one sees is the state of that star a bit more than four years ago. The star Betelgeuse is about 548 light years away. If it went supernova 500 years ago, we wouldn't know that for another 48 years. What we see on Earth is the state of that object when it emitted / reflected the radiation rather than the state of that object "now".
You mentioned expansion of the universe, which is why I wrote "now" in quotes. The concept of "now" becomes a bit fuzzy when relativity and expansion of the universe are brought into play.
Reading up on the James Webb telescope and how the images from it show us the universe as it was 13.5 billion years ago has made me wonder what does this mean?
It means that the JWST will be looking for electromagnetic radiation that was emitted when the first stars and galaxies formed, which was about 13.5 billion years ago (or maybe even a bit older than that).
And if the universe is always expanding does that mean there is more behind these images?
The universe hasn't always been expanding. One of the key concepts of big bang theory is that the universe formed about 13.8 billion years ago. The universe became clear about 370 thousand years after the universe formed. Before this, the matter in the universe was fully ionized, too dense, and too hot for electromagnetic radiation to travel very far before being absorbed (and re-emitted). We can see the signs of this clearing in the cosmic microwave background radiation. The few hundred million years between the this epoch (which we can see, in the microwave) and the formation of the first stars and galaxies is called the cosmological dark ages.
The JWST will not be looking into those dark ages, or before. It will be looking for those first stars and galaxies.
We're only seeing the "past" as it relates to photons. Yes, you are seeing the light from photons as they left that object when it was 13.5 billion light-years away, but the light from that object will have to travel much further to reach us due to that spatial expansion and recessional velocity, making it older than 13.5 billion years old by the time it reaches our sensors. For distances over 1-million light-years, the Hubble constant must be introduced to account for that expansion. And for distances smaller than that, the change is so small as to be negligible.