Since Nov 17, 2015 you have to authenticate users to make any (even such as "get some pictures who have specific hashtag") requests. See the Instagram Platform Changelog:
Apps created on or after Nov 17, 2015: All API endpoints require a valid access_token. Apps created before Nov 17, 2015: Unaffected by new API behavior until June 1, 2016.
this makes now all answers given here before June 1, 2016 no longer useful.