I need to represent an non physical enemy ( I can't just take a picture of somebody I don't like ) but I don't have any ideas on what to do, can anybody help me?

Non-physical enemy? Does physical mean fatty fast foods?

If physical means real, then you cant take a picture of it. So you will have to use symbols.
Here are some symbols:
Burning cross.
Picture of a screaming frightening woman.
Picture of a drink with a jug of wine next to them.

Now if you are allowed to use words, as in a story, you can paint some strong words....full of hate...full of fear...full of retribution....
I am reminded of one of Robert Brownings poems that does that...

http://www.sparknotes.com/poetry/browning/section2.rhtml