No need to learn them off, obviously learn about two or three phrases for each picture in the sequence. You won't remeber all twenty. The examiners tend to focus on the fluency of your delivery of the phrases (does that make sense to you). This is how I did it for my oral and it went alright.
Certain phrases can be used for multiple sraith pics, like saying that "the sun is in the sky", or "they were in front of ___", and it's the same for the questions you ask at the end, you just need to know a couple questions that you can use for any sraith pic, like "where were they at picture 1" or "what was the weather like in picture 3"
have about 5 or so key words and phrases for each and try use general language for the rest! Better keeping it mildly simple and not making mistakes than learning them off and making silly mistakes that show you dont actually know what you're saying
I took one a week from the start of 6th year, after that I just made sure I was comfortable with all 20. Its not as bad as it seems alot of the pics are similar and overlap with phrases and seanfhocails