Take a look here in order to get started: http://instagram.com/developer/
and then in order to retrieve pictures by tag, look here: http://instagram.com/developer/endpoints/tags/
Getting tags from Instagram doesn't require OAuth, so you can make the calls via these URLs:
GET IMAGES
https://api.instagram.com/v1/tags/{tag-name}/media/recent?access_token={TOKEN}
SEARCH
https://api.instagram.com/v1/tags/search?q={tag-query}&access_token={TOKEN}
TAG INFO
https://api.instagram.com/v1/tags/{tag-name}?access_token={TOKEN}