So, there's a few different things interacting here that are a little confusing.

First off, you have copyright law, which grants monopolies on the act of copying to the creators of the original. In order to legally make use of that work you need to either have permission to do so (a license), or you need to own a copy of the work that was made by someone with permission to make and sell copies (a sale). For the purposes of computer software, you will almost always get rights to the software through a license and not a sale. In fact, there is an argument that usage of computer software requires a license and that a sale wouldn't be enough because you wouldn't have permission to load it into RAM[0].

Licenses are, at least under US law, contracts. These are Turing-complete priestly rites written in a special register of English that legally bind people to do or not do certain things. A license can grant rights, or, confusingly, take them away. For example, you could write a license that takes away your fair use rights[1], and courts will actually respect that. So you can also have a license that says you're only allowed to use software for specific listed purposes but not others.

In copyright you also have the notion of a derivative work. This was invented whole-cloth by the US Supreme Court, who needed a reason to prosecute someone for making a SSSniperWolf-tier abridgement[2] of someone else's George Washington biography. Normal copyright infringement is evidenced by substantial similarity and access: i.e. you saw the original, then you made something that's nearly identical, ergo infringement. The law regarding derivative works goes a step further and counts hypothetical works that an author might make - like sequels, translations, remakes, abridgements, and so on - as requiring permission in order to make. Without that permission, you don't own anything and your work has no right to exist.

The GPL is the anticopyright "judo move", invented by a really ornery computer programmer that was angry about not being able to fix their printer drivers. It disclaims almost the entire copyright monopoly, but it leaves behind one license restriction, called a "copyleft": any derivative work must be licensed under the GPL. So if you modify the software and distribute it, you have to distribute your changes under GPL terms, thus locking the software in the commons.

Images made with software are not derivative works of the software, nor do they contain a substantially similar copy of the software in them. Ergo, the GPL copyleft does not trip. In fact, even if it did trip, your image is still not a derivative work of the software, so you don't lose ownership over the image because you didn't get permission. This also applies to model licenses on AI software, insamuch as the AI companies don't own their training data[3].

However, there's still something that licenses can take away: your right to use the software. If you use the model for "commercial" purposes - whatever those would be - you'd be in breach of the license. What happens next is also determined by the license. It could be written to take away your noncommercial rights if you breach the license, or it could preserve them. In either case, however, the primary enforcement mechanism would be a court of law, and courts usually award money damages. If particularly justified, they could demand you destroy all copies of the software.

If it went to SCOTUS (unlikely), they might even decide that images made by software are derivative works of the software after all, just to spite you. The Betamax case said that advertising a copying device with potentially infringing scenarios was fine as long as that device could be used in a non-infringing manner, but then the Grokster case said it was "inducement" and overturned it. Static, unchanging rules are ultimately a polite fiction, and the law can change behind your back if the people in power want or need it to. This is why you don't talk about the law in terms of something being legal or illegal, you talk about it in terms of risk.

[0] Yes, this is a real argument that courts have actually made. Or at least the Ninth Circuit.

The actual facts of the case are even more insane - basically a company trying to sue former employees for fixing it's customers computers. Imagine if Apple sued Louis Rossman for pirating macOS every time he turned on a customer laptop. The only reason why they can't is because Congress actually created a special exemption for computer repair and made it part of the DMCA.

[1] For example, one of the things you agree to when you buy Oracle database software is to give up your right to benchmark the software. I'm serious! The tech industry is evil and needs to burn down to the ground!

[2] They took 300 pages worth of material from 12 books and copied it into a separate, 2 volume work.

[3] Whether or not copyright on the training data images flows through to make generated images a derivative work is a separate legal question in active litigation.