Second, it is easy to prove that an AI company used my pictures, since they would need to download the pictures to train on them.
The problem is the AI isn't going to use exactly the original image it will take parts of it and use it to create a new image that does not include the metadata. Just proving they downloaded it wouldn't be actionable since they would need to download the image to get your metadata which contains the terms. You would need to prove that they applied the transferred the data to the training algorithm which would be an internal process.
Also there's the concept of consideration to... uh... consider. Is a million dollars reasonable consideration?
But that being said, it's worth a shot. Worst case is you get nothing, but you'd get nothing if you didn't put licensing terms in the metadata anyway. Just seems very unlikely you'll get a million dollars by doing this.