Board Thread:Support Requests - Designing Your Wiki/@comment-5556688-20121216211604/@comment-3448772-20121218200509

I don't know for sure, but I think that they did this.

So first thing is that when you upload an image for the achievement an image file on the wiki is created to store it.

I assume that if you change that image it won't add the border, however I think it will still re-size so you have to make sure that the images are the right size.