I recommend you to check the following files in the FEX:
http://www.mathworks.com/matlabcentral/fileexchange/22264-mosaic-random-images http://www.mathworks.com/matlabcentral/fileexchange/7876 http://www.mathworks.com/matlabcentral/fileexchange/31279-createmosaic-m http://www.mathworks.com/matlabcentral/fileexchange/30039-mosaic-generator http://www.mathworks.com/matlabcentral/fileexchange/636-photomosaic
