It appears you are not restricted by the use of media but your problem may be availability and what you may have at home at the moment. However the closing date is in July.
Possible areas to research are botanical images, Michelangelo's studies of skulls and Georgia OKeeffe’s stylised drawings of animal skulls and flowers, Monet and Van Gogh’s drawings of flowers, and paintings by Janet Fish.
If possible it’s best to draw from life but again could be quite restricting being at home however play to your strengths.
Year 10 students, as well as possibly winning the competition, any work produced can obviously go towards your GCSE course work. Good luck.