vaulty app question

ive got the nexus 7 and love it,ive just downloaded vaulty to protect some pics from my wife,if you know what i mean lol.it seems simple but im so frustrated,the pictures are in my dropbox on my nexus,so i bring up the pic,press import and vaulty is there,i press pic to go there,seems simple.but i get message saying it coudnt be done and it hasnt worked.i dont understand why,im new to the tablet so now im getting frustrated,anyone able to step me through this please.
cheers john