Upload your pic to one of the many photo-hosting sites - I use muchos and photobucket - thse are both free of charge. Make sure the photo is not too large.
Then go into your profile by clicking 'profile' near the top of the CA page, then click on 'Forum Profile Info'. In there is a 'I have my own picture' button. Check this button, and add the URL of where your piccy is. Save the amendments, and Bob's your Auntie.
Del
Edit: To give you some idea, my Avi is 200 by 114 pixels at 300dpi. Del