Darek - perhaps create the 64x64x3 matrix with all elements set to 255 and then replace a subset of that matrix with the 50x50x3.
myImage = uint8(randi(255,50,50,3));
myPaddedImage = uint8(255*ones(64,64,3));
myPaddedImage(1:50,1:50,:) = myImage;
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!