We always need at least 3 photos of you wearing the piece. These become your page's hero image and gallery. Three to six is plenty — pick your best. If you're posting a carousel and want the whole set on your page, you can add up to 20. Max. 20 images. For your published post, remember the "headline" on the last slide "What is your word? Follow link in bio for 15% off". Not needed here.
iPhone photos are fine. They convert automatically when you upload.
On Android, if your photos look greyed out they're saved as HEIC. In your camera settings turn off "High efficiency pictures", or open the photo in Google Photos and share it from there — it saves as JPEG.