If you know the aspect ratio that you want your photo to be, you can use photo editing tools to crop your photos to the correct size.
Check out this article for information on how to figure out the aspect ratio you need:
Once you figure that out, you can use the built in tools in the iOS photos app.
You will want to select the photo you want to edit, and then you will tap the crop button:
You'll then want to tap the aspect ratio button:
From there you will get the different options to crop your photo:
Once you choose the correct size and finish cropping your photo, you can tap done and put the photo in your Universe Site!