Continue gluing seeds a section at a time. Use different seeds for each section.
You can use a small spoon to put the smaller seeds over the glue. Do not worry about putting seeds over the outlines of the shapes. Once the glue dries, you can slide the excess seeds off the paper.
A border made of seeds is fun to make and looks quite charming too. It adds a nice touch to a small and simple mosaic picture.
Besides nut shells, you can also accent your seed mosaic with spices like star anise and cinnamon sticks. They look quite nice and smell nice too!