You have dimensions of each image...say dimensions of each image [m,n,3]. Initiate a matrix idx such that, each quarter has the value 1, 2, 3, 4 for each respective image. When you get [x,y] use interp2, and get the value.
idx = zeros(2*m,2*n) ;
idx(1:m,1:n) = 1 ; % first image
idx(1:m,n+1:end) = 2 ; % second image
idx(m+1:end,1:n) = 3 ; % third image
idx(m+1:end,n+1:end) = 4 ; % fourth image
p = interp2(idx,3000,2000) % see the value and decide the image
