If you have the latest version of Yuku ... follow this method for a new avvie:
1. Click on the 'Profile' icon at the top of the screen (where it says "Welcome ..." and your name)
2. Put your mouse over the "About Me" box just above your current avvie and it will show up 'Edit' ... now click on the 'Edit'
3. You should have a new screen similar to the one you've just left ... with the message bar saying "Edit 'About Me'"
4. Click the little box next to your current avvie where it says "Upload New Photo"
5. Follow the prompts from there.
I uploaded my photo directly from one stored in my 'Pictures' area of the PC. Or you may upload it from a Photobucket or a similar photo storage programme.