Check out the tutorial page at this site: http://www.gardenhose.com/, look for the one on "Painting Tiling Texture Maps". Even if your paint program dasn't support "tiling" you can paint the foliage (or use a scan), create an alpha channel for the transparency and map them to a mesh. It's really easy with the leaf nozzels in Painter but most paint programs will at least let you build a custom brush to achive the desired leaf-group brush.