can I use instagram pictures for an academic work The question is pretty easy and fit in the title.

So: can I use instagram pictures for an academic work?

I think I have to ask people who posted them if they agree but I'm not sure. I don't want any trouble with plagiarism and copyright.
In a a word no, users of Instagram retain copyright of their own images, and grant Instagram a licence to reproduce them on Instagram's website. No licence is granted to other users of the website, so you would need to approach each user individually to gain permission, unless your usage comes under a fair-use right. This differs between countries, so it depends on where the user is located that posted the image, and where you are located as to what rights you may have under this. These fair-use rights are very restricted, and have no relationship to academic work in particular.