If u want it to b positive u can show a beautiful landscape with a waterfall perhaps or a river or pond or any other water body which is clean. U can add some life forms to make it look more positive. Add marine life in the landscape and other animals near the water.Most imp..... write a slogan with bold letters all over the sheet or on the top of it. the slogan should emphasis on saving the water.
If u want a negetive picture do jst opposite of the positive one. dirty water, dead fishes and no other life forms. N yes a warning slogan.It can b Water is life, dont polute our lives.
Remember negetive is more influencing than the positive one
a picture that might help u: