- Check the format is JPG, PNG or WebP. An iPhone HEIC will fail.
- Check the size. Photos straight off a modern camera can be 10MB or more; resize the long edge to about 1920 pixels.
- Try one image at a time rather than a batch.
- Try a different browser, or a laptop instead of a phone on a weak signal.
A slow mobile connection dropping mid-upload looks exactly like a broken uploader. Retry on wifi before assuming the worst.