I am not sure how a togglethumbnails button in the general imagebuttons can be more obvious than a separate CLOSE button located in the top right of the image. Anyway, you can add togglethumbnails button if you like by doing this:
Open imagevue/include/config.xml, and find the line:
<buttons type="array" description="Select which buttons to be used when hovering over the image" options="imageinfo,window,popup,copylocation,link,purchase,download,sendlink,contact,zoomin,zoomout, togglefullscreen">window,popup,download,zoomin,zoomout</buttons>
Change it to:
<buttons type="array" description="Select which buttons to be used when hovering over the image" options="imageinfo,window,popup,copylocation,link,purchase,download,sendlink,contact,zoomin,zoomout,togglefullscreen, togglethumbnails">window,popup,download,zoomin,zoomout</buttons>
As you may be able to see, I have added "togglethumbnails" in the options array. Save and upload this file, then go to your admin -> settings, and locate this option. You should now be able to add a "togglethumbnails" button to your imagebuttons.