There aren’t cropping or other image editing tools built into the website (see related feature request), so you’d need to remove and replace the image on your observation with an image you cropped/edited elsewhere.
iNaturalist doesn’t to my knowledge crop images, it just resizes them - can you provide an example original image you uploaded and URL of the observation where it looks like the website uploader modified your image?
Not relevant to you, but for others: there is a cropping tool in the Android app, and bringing iOS up to feature parity with Android is on the team’s to-do list.