On the passport photo page,
https://travel.state.gov/content/travel/en/passports/how-apply/photos.html , about 1/3 of the way down there is a tab 'already have a photo?' which takes you to this page:
https://tsg.phototool.state.gov/photoThere you can upload your photo, looks like it will automatically crop it, or you can manually crop it by placing the eyes in the correct spot and then it sizes the crop correctly in proportion to the face. When done, you download the file to your device.
I'm doing this on a laptop-but it looks straight forward and effective.
Edit to add-just tested it on my iphone (15 Pro) and it worked. After uploading the picture, you let it auto crop (worked well), or manually, then click the 'download to device' button. Had to go into my downloads and then 'save image' to my photo library.
