You can use your own icons or the icons from the free pack =)
You get to choose which icons are used for which achievement etc, and you can place the icons in the relevant folder to use them for trophies etcetera.
There is also the option of purchasing the premium pack of approx 250 icons which you can use in this way also =)
Just to add to what Cosmic said: The folders you need to upload images to are also described next to the dropdown.
So for instance, for Achievements it'll say /images/icons/vbactivity/achievements is the folder to put images in.