That must have come as a shock. You have never really thought that your profile picture on twitter is an important thing and now you are being told that it is the key to getting your 500 twitter followers? Well, you need to believe it and start working on that profile picture.
The profile picture ought to be personal. People need to see and know whom they are interacting with. They need to relate to you as a person. This simply means that there is no option of putting a profile picture of a quote or an animal or a painting or any other idea that you might have. Let people see you, let them know who you are and what you look like. The moment people personify you, it will be much easier for them to follow you and you will easily get your 500 twitter followers. Make sure that you put a mature profile picture. You want people to relate you in a certain way, the picture that you put there should project the image of a person who can be related to in that way. If you want people to respect you, place a respectable profile picture, if you want people to find you funny, place a funny profile picture and so on and so forth.