Is it illegal to post a team picture that you didn't buy on instagram?

So online my team pictures were posted for previews with like the company logo (in focus photography) on it and if you wanted to buy them you can buy them online. They are so expensive tho! But if i used my iphone to take a pic of the computer screen, would that be okay to post on Instagram?

No, that is a copyright violation. You could be sued by the photographer if you get caught.

No, that's a clear copyright violation. You don't have the right to make and distribute copies of another's work.

In this case, a photographer has provided "proofs," since you need something to look at for follow up and buying things. You don't have any right to those images, and this is very clearly a professional expecting you to follow up and buy things. You know what you're supposed to do. Doing something like taking a picture of their image on the screen is no creative work on your end, and would be clear copyright infringement -- doubly-so because you're then posting the image on instagram to distribute it.

If you like the pictures, you need to find a way to buy them. If you think the prices are too high for some reason, you can maybe talk to the photographer to see if you can arrange a discount, like if lots of people from the team buy things. Otherwise, your infringement ensures the photographer doesn't get paid -- despite you liking their work -- and they have no incentive or ability to do work in the future.

Anyway, no, what you're proposing isn't okay. It's flagrant copyright infringement, and a fairly direct reason of why we have copyright protections to begin with.