If you know beforehand which frames you want to keep then store them in a vector. Let's say your movie is in a cell array, then you can access every frame of interest by referencing to the vector directly.
1) YourMovie is a cell array containing 20 frames. 2) You want to extract frames 2,5,7,8,10 and 15. (arbitrary frames).
Create the vector for the frames you want to keep:
FramesVector = [2 5 7 8 10 15];
ExtractedFrames = YourMovie(FramesVector);
ExtractedFrames will give you every corresponding frames in a cell, which you can then access individually like you would any cell array: