If you share your photos to Instagram, you have to crop it in some cases. Instagram still has limitations on what non-square aspect ratios you can use.
For landscape (horizontal) photos, you can use aspect ratios up to 1.91:1, for portrait (vertical) photos you can use an aspect ratio up to 4:5.
So if Pixbuf wants you to crop your image, it means that it is not in these aspect ratios. Currently, we can drop only square photos, but we plan to enhance our crop functionality soon.