To be sure I'm clear here, I'm going to try and paraphrase. Please let me know if I've got it!

You need 8 captions per picture. Caption1, Caption2, ..., Caption8. These captions are constant for all of the images but will be presented/arranged in a random order for each of the 30 images.

Selecting the Captions:

Each caption is randomly selected from a specific set designed for it so, e.g.,

Caption1 will be chosen randomly (assuming with replacement here since it is only done once for each set) from a set such as Caption1a, Caption1b, Caption1c, Caption1d.

Caption2 will be chosen randomly from a set such as Caption2a, Caption2b, ..., Caption2i.

Then, once the 8 constant captions have been determined and then randomly ordered for each image, we randomly order the 30 images for presentation.

Yes? No?