Why photos show up sideways
Smartphones save photos with the pixels as they are and record only "rotation info (EXIF Orientation)". Programs or sites that can't read this show the photo sideways. Rotating the actual pixels with this tool makes it upright everywhere.
When you need a horizontal flip
Selfies taken with the front camera are mirrored, so text can look reversed. A horizontal flip restores the original direction.
Photos are never uploaded to a server — they're processed only in your browser. Safe even for personal photos, IDs and documents.