Four pictures one word help?

Http://s1169.beta.photobucket.com/user/Jack_Ballard/media/photo.jpg.html?sort=3&o=0

The game is for Kindle fire and me and my mum have both been stuck on this for hours!

I play this game and this one seems hard don't know I thought maybe stack but there's no s.

Compound?

COMPOUND

Try compound