The question is "what purpose does the feed serve"? If the page would be equally valuable to all users without the feed, then it can be judged to be presentational and null alts would be acceptable.
If it is not presentational, then if it is your Instagram feed, or you have the ability to influence the curator of the feed (e.g. you are paying for it) and there is the ability to enter a text description of the image that you could put into the alt
attribute (or put inside an HTML5 <picture>
element https://developer.mozilla.org/en-US/docs/Web/HTML/Element/picture), then you should do so and failure to do so would constitute non-conformance.
If it is not presentational and you cannot control or influence the content but you can take content that (should it exist) and place it in the alt
or a <picture>
element. Then you should do that.