Storing image properties in for-end loop

I would like to ask you to help me to store image name and map in for-end loop. Here they are in square brackets [X, mapX] but how to store for each image name the properties in different letter?

a = dir(fullfile(yourfolder, 'img_??.jpg')); 
for i = 1:length(a)
    imgname(i) = char(a(i.name)
    [X, mapX] = rgb2ind(imread(imgname),256);
end ;

Best regards Michal