As well as Page Up and Page Down, you can use Space and Backspace, or the mouse wheel, or left-click. (The last two are options in Opus's Preferences.)
You can also use a tool like AutoHotkey or AutoIt to give the viewer alternative hotkeys.
If you install AutoHotkey and add this to your AutoHotkey.ahk config file then the cursor keys will change images in the Opus viewer:
#IfWinActive, ahk_class dopus.viewpicframe
Left::
Up::Send {PgUp}
Right::
Down::Send {PgDn}
#IfWinActive
2019 edit:
You can edit the viewer's hotkeys these days, so you don't need to use AutoHotkey or similar for this anymore.