The mirror effect nobody asked for
Take a selfie with text visible somewhere in frame — a t-shirt logo, a sign in the background, a book cover — and it's a coin flip whether that text reads correctly or backwards in the saved photo. This isn't a bug, exactly: many front-facing cameras display and sometimes save a horizontally mirrored version by default, because a mirror image feels natural while you're framing the shot (it moves the way a real mirror would). The trade-off is that anything with actual text or asymmetric detail comes out reversed.
A horizontal flip corrects this back to how the scene actually looked in real life — the fix is fast, but only if you flip in the right direction, which is worth understanding rather than guessing.
Flipping and rotating look similar but aren't
It's easy to mix these two up since both change an image's orientation, but they do fundamentally different things. Rotating spins the image around its center point while keeping its left-to-right reading direction intact — rotate a page of text 90 degrees and it's sideways, but every letter is still correctly shaped, just tilted. Flipping mirrors the image across an axis, reversing that reading direction entirely — flip that same page of text and every letter becomes backwards, like reading it in a mirror.
This is exactly why a mirrored selfie needs a flip, not a rotation — rotating it any amount would never fix backwards text, since rotation never reverses the underlying reading direction that's actually broken.
What happens when you flip both directions at once
Flipping horizontally and vertically at the same time produces the same result as rotating the image 180 degrees — every point in the image ends up diagonally opposite from where it started. This is a genuinely useful shortcut to know if you ever need a 180-degree turn and only have flip controls available, though a dedicated rotate tool is more direct when that's specifically what you're after.
One thing flipping won't fix
If your source is an animated GIF, flipping typically only affects a single frame rather than the whole animation, since flipping every frame correctly requires processing each one individually rather than treating the file as one static image. If you need a flipped animated GIF, look for a tool that specifically handles animated content frame-by-frame rather than a general-purpose static image flipper.
The same caveat applies to any other multi-frame or layered image format — a static-image flip tool is built around the assumption that a file represents one visible frame, and anything with additional hidden frames or layers needs a format-aware tool instead.