There are two separate questions here:
1. Is the output of a program copyrightable?
2. If so who owns he copyright?
We can work from a more familiar example.
1. A compiled binary program is widely accepted as being copyrightable. If that were to be overturned the entire packaged software industry is in trouble. The binaries are the output of a compiler or maybe a linker so the output of a program is copyrightable.
2. Who owns the copyright of the compiler/linker output? That's fairly clear (but see below); it's primarily the owner(s) of the copyright of the source code that was fed into it.
We have to go round that loop again in that the source code itself will be the output of one or more editing programs. The ownership of that might be a bit more complicated but the legal principles are well enough established. It could be whoever was pounding the keys. However where that was done in the course of employment then it will normally be the employer. This is all established stuff, you just have to look at the context.
There's something of a complicating factor in that the vendor of the compiling system may have provided utility libraries which are linked in as may some third parties. Their ownership (established as above) and the terms on which they're provided may also have to be taken into account. Again, this is all old stuff.
If we now transfer this to the AI situation it should become fairly clear.
1. The image is the output of a program and, as we've established, subject to copyright.
2. On the analogy above the output must be owned by the owner(s) of the input. The inputs appear to be the selected description and the training data. The description seems to have been provided by the operator but who owns the training data? Is this provided by the operator or is it owned by one or more additional parties and if so under what conditions?
Having thought this through I'm wondering if these attempts to credit authorship to the AI are aimed at getting round thorny questions relating to the provenance of the training data.