Image overlay with matrix
%# some image
I = im2double( imread('peppers.png') );

%# I create here a random mask (gaussian centered in middle of image)
[r,c,~] = size(I);
[X Y] = meshgrid(1:r,1:c);
Z = mvnpdf([X(:) Y(:)], [r c]./2, diag(15.*[r c]));
Z = (Z-min(Z(:)))./range(Z(:));
Z = reshape(Z',[c r])';

%# show image and mask separately
subplot(121), imshow(I)
subplot(122), imshow(Z)

%# show overlayed images
figure, imshow(I), hold on
hImg = imshow(Z); set(hImg, 'AlphaData', 0.6);

%# also we can specify a colormap
colormap hsv