To add your picture to your PGP key using GnuPG on Linux use:
List the certificates:
Find your and yours certificate will start with:
pub 1024D/XXXXXXXX YYYY-MM-DD
Use this command to edit your key:
gpg --edit-key XXXXXXXX
Add your picture using:
You must use a JPG photo. The recommended size is something near 240x288. If you use big images, your certificate will be HUGE.
Add the path to your photo:
The system will ask for confirmation, double check it. And press Y if it is correct.
Type your password when the system ask for it.
Save the modifications
Send your key to a key server.
gpg --keyserver keyserver.cais.rnp.br --send-keys XXXXXXXX
That is it.