Am I allowed to use an apple device from placeit.net with an app screenshot on Twitter?
You may want to check this answer:


  You will get lots of free iPhone 4s images from internet. There are
  lot of sites which provides freebies for designers here is a link
  
  365psd.com www.pixeden.com
  
  BR, Hari


If you are part of the apple developer program you can access the marketing section and get the iphone images that you are able to use.

Hope it helps.