Use a program, e.g. this one http://www.base64encode.org/
to encode the achievement url and add it here:
the problem is the /U2/9D/ part = certain letters/numbers referring to the image path on xbox.com. Atm I don't understand the logic behind them and as long as you don't know the correct path you won't be able to get any tile
Furthermore I'm not sure about the numbers after /ach/
I think they have s.th. to do with grey/colored version of the tile.
usually the numbers increment but there seem to be lots of exceptions.
It's complicated and currently I don't know 100% how it works, but here is some info:
I think it might still be possible but MS is making this harder and harder. Basically it's similar to grabbing the game tiles and gamerpictures of image.xboxlive.com.
You need the titleID of the game (click here for info)
and this is the link for achievement images:
Trading with different people should increase your chances or importing from different games since you will have more Pokemon with different ID's that way. I'm pretty sure it's for every Pokemon you have (Party and your PC).
You have a better chance of winning something from the lottery by having Pokemon with different ID numbers, and you can do it once a day. Look at the second thing listed here for how the lottery works.