Mouse wheel always switch image,even if it's set to a different action.
It is not possible to change this behaviour in 0.3.0 2 actions are available: Mouse-wheel: switch image Mouse-wheel+Ctrl modifier: zoom in/out. They are non-configurable.
Oh sorry! It's my fail! I think it option is change mouse scroll action. In Russian translate writing is similar. Maybe necessary to make such a function-zoom images by mouse scroll without CTRL.=)
This behaviour was changed to make both features available to the user without having to choose for one or the other. Switching this behaviour does not make sense, zooming with ctrl-mousewheel works for file-managers, webbrowsers and image-viewers. There is no need to change that.