With me it works in either one of two ways which are the following:

1. I write what they are about before finding a picture yet I do have a certain idea of what they would look like roughly. I tend to write characters in all aspects which do create unusable characters which hardly fit into anything though that happens more in the second way I create characters then my primary way. A picture does paint a thousand words but sometimes fleshing them out before getting a picture helps me more than with one as it limits me to the build they most likely have, the age and so on.

2. I find a picture online randomly and a character starts to come to life in my head where I proceed to write the character out. First in a basic form and then I begin to flesh it out. This is the less used route but when it happens I make sure to get all I can down before the idea leaves my head. Time to time I have been out and about where I see an image via phone or something but can't do anything about it. I have some unique characters I have never used or ever get to use because of this.

Unless I am joining an RP my characters are set in some way or they aren't. I rarely make characters for genre based RP's made by others so I adapt them or if I have to make them. They are usually set in the way I want them which as an example set for a gang based RP, a horror RP, Supernatural and so on.