If the CMB is an image of the universe when it was 379,000 years old, how is it so large? Since the light is 13.8 billion years old, the image represents the surface of a sphere with a radius of 13.8 billion light years, and I don't think the universe would have been that large at that age. Even if it was, an image from further back in time would be larger, which doesn't make any sense, since the universe expands.
The CMB isn't really an image of the universe. The universe is full of the CMB radiation, with (almost) equal numbers of photons travelling in all directions at every point in space. An analogy would be taking a picture of fog from within the fog.
We are immersed within the cosmic microwave radiation field and our position (and every other position) in the universe has always been fully immersed in this radiation field.
All those photons have indeed been travelling from elsewhere in the universe since they were created 13.8 billion years ago. The regions from which those photons originate are now, thanks to the expansion of the universe, about 46 billion light years away. The uniformity of the cosmic microwave background tells us that the universe is very homogeneous on scales at least as large as this.
i.e. The CMB photons we see now come from a sphere around us that is now (at this cosmic epoch) 46 billion years in radius. In a billion years time they will come from a sphere around us that is correspondingly larger.
The size of the CMB sphere is not related to the size of the universe. It could be that the universe is infinite in size, and was infinite in size when the CMB was emitted. It could also be that the universe is smaller than the CMB sphere; that's just another way of saying that CMB light could have circumnavigated the universe at least once between emission and detection. I think the latter is ruled out experimentally, but it's possible in theory.
I think it's reasonable to say that the CMB is an image, but it's not an image of the whole universe, just a spherical slice of it. The universe isn't (and wasn't) spherical in shape. If the spatial curvature is positive, it's a hypersphere, not a sphere, with a radius that's unrelated to the CMB radius.
Your argument that the CMB radius should be 13.8 Gly only works in flat spacetime and inertial coordinates. In fact the radius of the sphere that emitted the CMB was about 42 Mly. At the present cosmological time, it should have expanded to a radius of about 46 Gly, but we can't see that. The ratio between those values is the redshift (1+z ≈ 1100).
When people talk about the universe having been small in the past, they're properly talking about only the part of it that's currently observable. That was much smaller in the past, but the universe as a whole appears to be infinite and always has been. The expansion of the universe doesn't mean the total size gets larger (that's meaningless if it's infinite), it means all the stuff is moving apart (along with the geometry of the universe itself). The big bang occurred everywhere at once.
Over cosmological time, the CMB shows an expanding slice of the universe. The age of the universe when the CMB was emitted will always be the same, but the slice will get larger and larger.
From some comments of yours I don't understand when you say you want to know about 'the image' and not about the photons. The images we create are from the photons. 'The image' doesn't exist as a separate thing. The origin of the photons of the CMB as it would be seen from any point in the universe was a cloud of plasma at about 5000k that was the entirety of the universe when it was 379,000 years old. Different locations in our universe would see different images. A galaxy 100 million light years in one direction from us will see what we are seeing now in the opposite direction in about 100 million years, but shifted to even longer wavelengths.
The 379,000 years you mention is the time when the universe became transparent to photons of any sort, and that is the main way for us to observe it and what the CMB is. The thing is that since the universe is expanding. The CMB we see, and thus the images we create from it, is always from the time when the universe was 379,000 years old. Before that the universe was opaque so any photons would have been absorbed and re-emitted. Once the universe cooled enough to become transparent, photons were free to move about and we can gain information on what produced those photons by observing them.
As time goes by when we look at the CMB we are seeing further across the universe as it was at that time. Photons from parts of the universe that were closer to us would have long passed us by. Photons from parts of the universe that were further away from us are still traveling towards us. Photons from parts of the universe much further away will never get to us because the universe is expanding too quickly. We can also see photons from closer objects that generated or reflected them after the universe became transparent, but that is not the CMB.
Right after the universe became transparent the photons would not have been microwaves, they would have appeared with shorter wavelengths. This answer seems to say they would have been around 5000k, which is visible light. It's complicated as there was a long period before reionization when the universe was still opaque to visible light at large scales.
See my answer here on physics for more information.