With the new feature to publish directly to Instagram via Hootsuite, the images you upload will need to be within the requirements imposed by Instagram, available here: https://help.instagram.com/1469029763400082.
Please also keep in mind the minimum resolution is 150x150 px and the maximum resolutions are:
for landscape images: 1080x566 px (1.91.:1) for square images: 1080x1080 px (1:1) for portrait images: 1080x1350 px (4:5)
Hope this information helps!
Alina | Customer Advocate