Putting together a design that looks and feels right is probably more difficult than it seems. There are plenty of copyright free line drawings of flowers in, for example Search Press books and Dover Publications. Also photos can be transferred to drawings by using a grid of squares and transferring the outlines using the squares as guides. If you have and iPad you can get an app called 'copy it' which is a grid that can be sized up and set over your photo on the screen.
If you are creating your own flower pictures there is plenty of advice in art books on composition. It is however, important to group flowers in odd numbers as this is most pleasing to the eye. A triangle shape can be used as a basis for a collection of flowers making sure that larger blooms are at the base to balance the design.