When requesting rights to use content, there are a few types you should avoid. 

Professional photographers
Professionals typically expect compensation for their work beyond having it featured by the brand or business. If you request rights, be prepared to negotiate.

Photos with identifiable people (other than the photographer)
Unless the photographer has a model release, photos with clearly identifiable people should be avoided.

Photos taken by minors / children
If the Instagram user appears to be under 18, their content should be avoided. Tip offs may include posts of a recent school events.

Photos that are not original content / "hub" accounts
Some Instagram users repost photos (with and without attribution). Sometimes they use a tool that marks the photo as a repost. Other times, they just downloaded the photo from somewhere and uploaded back to Instagram. There are a number of "hub" accounts that ask users to tag their photos with a particular hashtag to be featured. There are also individual users who will occasionally take and post a photo that they like. They usually do not own the rights to these photos and cannot grant any rights to you. Always make sure to check to see if the caption or first comment contains a photo credit.
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