Opus Light does not expose those settings in Preferences; however, you can disable that toolbar by editing a configuration file.
To do so, type /dopusdata/ConfigFiles into Opus to locate the required directory.
Open prefs.oxc in a text editor (e.g. Notepad).
Close all Opus windows. (With the Light version, that should exit the process. Opus Pro users see here, but Pro users shouldn't need to edit the file directly in the first place.)
Fairly near the bottom of the file you will find this:
<viewmodes>
<mode mode="6" name="Images" set="no" type="0" />
</viewmodes>
Change it to this:
<viewmodes />Save the change, then re-launch Opus. The Images toolbar should no longer appear when switching to Thumbnails mode.