What part of it are you having difficulty with? I assume you loaded the 20 images from file in the first place so just load 2 instead of 20, then do the same again. I'm not sure I understand where the problem lies.
If they weren't originally from file save them to a mat file.
doc matfile
does actually allow you to work with large data sets from file as though they were in memory (though a little slower, of course, especially depending which direction you index into the data). Or just a series of individual .mat files for each image.
