Yuku doesn't allow hotlinking to outside AVS like ezboard did (the ones you see that are that way, are just default ones from the account migration over here). Here's how to put in your own avatar:
1) Go to Images (next to Account). There'll be a folder there marked Avatars. Click it. Once inside you'll see where you can upload a picture to use for your avatar. Just do as it says. The maximum size for a Yuku avatar is 100x100. If what you're loading is larger than that it'll get squooshed down. If it's smaller (such as the 60x60 ones we used at ezboard) don't worry: you can still use them. Yuku will fill in the 'dead space' so it still looks the same.
2) When you're done, click Account again and click whatever is in the box to the left of your controls (in this case, it's my own beautiful green 'froed countenance)
3) Now just choose from whatever avatars you've uploaded on the right, click the one you want to use, and viola, you'll be set!
As far as storage goes don't worry: Yuku avatar albums store 30 MEG of material -- so if my 700+ icons (amassing only 9 MEG) will fit, I doubt you'll be running out of space any time soon. Plus you can always delete the ones you don't want to use anymore.
That's it: so easy a caveman could do it!