There is a settings option to have the mouse scroll wheel either zoom or change the currently selected photo under "Settings > Configure gwenview > Image view" but this only applies if the mouse cursor is held over the picture currently being viewed.

I suggest adding an option under settings to be able to change the currently displayed picture by scrolling over the pictures in the thumbnail sidebar in Gwenview. This way a user can have the best of both worlds. Scrolling over the displayed picture will zoom in and out and scrolling over the thumbnails will change the picture. It's a win-win situation since you can have both functionalities available at the same time. (Note: if the cursor is hovering over the scrollbar instead of the thumbnails themselves then it should scroll the scroll bar instead without changing the displayed picture, so this will now satisfy all possible usecases for the user)

The current behaviour is that it will only scroll the scroll bar when you do this but without changing the displayed picture.

Operating System: Manjaro
KDE Plasma Version: 6.3.5
KDE Frameworks Version: 6.14.0
Qt Version: 6.9.0
Kernel Version: 6.12.28-1-MANJARO (64-bit)
Graphics Platform: Wayland