Photo editors help me with program

Heya guys, I’m trying to make a mosaic-style poster. One of those posters that when you look from afar, it’s a photo of something but when looked upon closely, it’s made up of tiny smaller photos. What program do you use for it or do I have to do this manually(seperate photos into different color groups and arrange them to make a photo)?

http://www.andreaplanet.com/andreamosaic/

or PictureMedley

http://blog.photility.com/labels/downloads.aspx

both are free
.