It is important to have control over the image. You need to be able to make the image do what you want it to do. The length of time it takes you to run the image is also important. If you are imaging yourself swimming 100 free in 59 seconds, then that is how long it should take you to run the image in your mind. Give your imagery time to work.
Imagery needs to be practised just like physical skills.
You might begin practising your imagery at home in your room before you go to sleep, where it is a nice quiet situation. But to transfer this to behind the blocks at Junior Nationals will be difficult. Start to practise your imagery in a quiet atmosphere and then move it up to different situations where there are more distractions. Your bedroom, the dinner table, in front of the T.V., on the bus or subway, in the hallways at school, at training, at a small competition, and then at a bigger competition. This will allow you to develop the ability to deal with distractions and a higher level of arousal.