I moved this to the Android Apps forum.
Word of caution -- questions like yours often attract a lot of opportunistic spammers trying to foist their terrible apps laden with adware and worse. Be careful with any app suggested here that isn't well-established from a well-known developer -- look through the reviews carefully, specifically for complaints about excessive or intrusive ads. Also pay close attention to the app permissions -- a photo collage app should not have need for Phone, Contacts, or Microphone permissions (unless there's a big social component to the app).