Yes, you’d use one image that represents the main-titles (along with the additional image that tells you what’s inside that main-title) and one image that represents the subtitles. If there are subtitles within a subtitle, you’d have to use yet another image for that.
For the subtitles you could for example use a submarine as image.
In laws, the subtitles are often numbered with roman numerals so I use a roman (like one of the Asterix comics) as an image for these kinds of subtitles:
There can be a lot of sub-sub-subtitles (and even more sub than that) in laws.
So what I do before starting to memorize, is to check how many levels of subtitles there are in that law in order to plan on how many images I need that represent the different subtitles.
Also: I’d take notes of what you put in the memory palace, which is very useful when you need to review the information. Spaced repetition is very important.
(I put appointments in my calendar to remind me of the reviews, with an estimated time that I need for the review).
How many images for the detail information you use, is up to you. It just needs to remind you of the information and doesn’t need to be word-by-word. I usually try to use as few keywords as possible because that makes it easier to memorize (and for recall later on) and doesn’t use too much space in the memory palace. There are some things that I have memorized where one keyword was not enough, but you’ll have to see for yourself how detailed your images have to be, based on the specific information because everyone thinks and memorizes a bit differently. If a heart and a yellow safety vest is not enough to remind you of “Principles of Health and Safety Management” then you can certainly add more to the image that helps you with recalling the information. Just don’t get too perfectionistic (I made that mistake in the beginning), it can get crowded and exhausting quickly. My advice is to keep the images simple even if they’re not 100% accurate, because you still get the meaning, even if you didn’t memorize the information word-by-word.
The more you practice with these memory techniques, the more efficient you get and you’ll see what works best for you