All stickers have a transparent background to allow them to be placed on messages and other images.
To access these stickers in iMessage:
1) Open the Messages App.
2) Choose a conversation from the list or start a new one.
3) Tap the ">" icon.
4) Tap the "A" icon.
5) Tap the four circles icon in the bottom left corner.
6) The Sticker Pack icon should appear in your iMessage App Drawer.
6) If you do not see the Sticker Pack icon, tap "Store" icon and then "Manage" tab to enable the Sticker Pack.