QuadViewer

Are you looking for an app to compare pictures side by side? This is exactly what you want!
QuadViewer is a multi-screen photo viewer that divides the app screen into up to four panels and allows you to view multiple photos at the same time. It's so simple, but intuitive and easy to use.
This app runs on both macOS and iOS, and is fully optimized for each platform.

----- KEY FEATURES -----
• 8 screen split patterns
• 4 split, 3 split, 2 split, no split
• Vertically aligned, horizontally aligned
• Operate on individual panels or all panels at once
• Select photos from the Photos app or the Files app
• Import from external storage and cloud services
• Sync/async operations between panels, such as zoom
• Swap the panels
• EXIF data display (Inspector function)
• Full screen display
• Share, favorite, delete and other actions
• Save bookmark information (Project function)
• Mouse gestures
• Multi-window support
• Dark Mode support

----- NOTES -----
What this app can't do:
• selecting photos by folder or by album
• editing images and metadata
• output combined photos
• interaction between devices