I need some guidance on were to / how to start, on implementing my own customized avatar function

What I'm going to do is to implement a dynamic avatar system. (it will fetch a fixed url string, as well as data from the db) "I'll explain"

This is the full url of the profile image

http://image.eveonline.com/Character/664309886_256.jpg

What i'm going to do are to strip it into parts.

Part 1: http://image.eveonline.com/Character/
(this value will never change)

Part 2: 664309886
This value will be fetched from the profile information on the user.

Part 3: _256.jpg
This one will change, according to the size.

How and were should i start?

And I also want this function to appear to the user as an option, as well as the default avatar for all users that have the required info.