(This text is based on machine translation from Japanese text.)
When the image in the page is sequentially examined on for speed-up, and it reaches outside the screen, processing is interrupted.
Therefore, it is likely to react to the image in a right and left side menu for the page of the composition, like left menu - main contents - right menu, etc.
These operation depend on the number of images in the page, and page composition.
In a present version, there is an item in which the setting change is not reflected until reloading the page.
// for Vimperator users: //
Use pass-through plugin like "feedSomeKeys_2.js".
/* example settings for feedSomeKeys_2.js */
Add following two lines to your .vimperatorrc file.
fmap -vkey -event=keydown <PageUp> <PageDown> <S-Space> <C-Space>,<Space>
I don't know the way to override or remap Space key, so use "Ctrl+Space" as "Space".