I = load('Image_names.mat') ;
names = I.Image_names ;
[F,N,E] = cellfun(@fileparts,names,'un',0) ;
iwant = str2double(N)
It is better to associate a number to the image, and append it to Feature vector. As the image names have already numbers, just pick them and append them. By the way, your feature vector attached here is of size 1000X121 and image names are 500X1.