I've upgrade to final release of macOS 26.0 (25A354). I'm using the (default) light theme on macOS 26.0.
When viewing images they all have now a 1-3 (?) pixel grey border surrounding them at all 4 corners of the screen.
This grey border was not existing on former macOS versions (no border visible).
Source image used
Zoomed in to the upper left (please ignore the white border, somehow my screen capturing tool added that one, focus on grey border)
Full screenshot:
