If HEIC won’t load in your browser, export to JPG/PNG first (on iPhone: Share → Save to Files, or edit & export).
Drag the photo to position it. Use the slider to zoom.
1080 × 1080 PNG

iPhone users: after tapping Download PNG, the image will open in a new tab. Tap and hold the image, then choose Save Image.

Privacy note: everything happens in your browser. No photo uploads to a server.