It really depends on what you like and find interesting. I would suggest to start by reading articles on Wikipedia and find a topic you like in image processing.
Go on your school's website and read about the labs, the professors and what their research topics are. Don't forget to read personal pages of PhD candidates. That should give you a nice overview of what is being researched.
Once you pick something you like you can go deeper and read about the state of the art for that subject, and then try to find something to improve, something you can contribute to.
Journals and conference proceedings are nice if you want to read about the latest research but it won't help you if you don't know what to look for.
Good luck!
Best Answer