I think a lot of practice is required, but I think the amount varies between competitors.
Ben Pridmore uses a 2-card system (Ben System) and places 3 images per location, so that only requires 9 locations. There’s a similar system called the shadow-system that requires fewer images.
If you make a 2-digit number system, you could practice with it for a couple of weeks and see if you like it.
It probably depends on the event. I think linking/stories can be used for the IAM images event.
There’s a thread for that part of the question over here: Abstract words
If you review them regularly, they should be pretty memorable.