I sometimes don't find a picture. Though when I do, it's almost always near the beginning.

If I'm not using a picture it's because I have a clear idea of the character appearance in my head, and cannot find anyone that would be able to suit the role. So naturally if I have a character concept I try to find a picture as soon as possible before my mind comes up with a mental image, and thus makes it difficult for me to find a suitable picture.

There have also been times that I have created a character concept based on an image given. So in those cases finding/having a picture would be the first step.