Macview

Open a picture, and that is all you see

No buttons, no browser. The window opens at the size of the picture, and the arrow keys take you to the next one in the folder. Measured here, a window is on screen about half a second after launch.

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Free and open source. For Apple silicon Macs running macOS 14 Sonoma or later.
The first launch has to be opened from the right-click menuSee how

The window opens at the size of the picture

What Macview does

Nothing on screen but the picture: No toolbar, no sidebar, no title bar. The picture sits on a plain dark ground, so its colours and detail are what you are looking at.
Whatever your Mac can read, it opens: JPEG, PNG, HEIC, WebP, AVIF, GIF, TIFF and the camera RAW formats. The reading is left to macOS itself, so there are no plug-ins to add.
Quick to open, quick to be rid of: About half a second from launch to a window. The app is 1.6 MB, it does not sit in the background, and Escape closes it.
Animations play as they were made: Animated GIF, APNG and animated WebP play at the speed written in the file.

Keys

→ Space: Next image
←: Previous image
Home / End: First / last
↑ / ↓: Rotate right / left
F / ⌘F: Mirror / flip
⌘+ / ⌘−: Zoom in / out
⌘0 / O: Fit to window / actual size
⌘O: Open a file or folder

Scrolling and pinching zoom as well. While the picture is larger than the window, dragging moves the picture; while it is not, dragging moves the window. A double-click fills the screen, and a second one puts it back. Files can also be dropped on the window.