Website homepage doesn't display correctly in Opera browser

Copy of this has been posted on the test server forum too.
I rarely use the Opera browser, but am using it now to check my own website for compatibility.

I've noticed that the main menu on the homepage looks like this:
http://i1146.photobucket.com/albums/...ps253e5c3b.pnghttp://i1146.photobucket.com/albums/...ps3b370a93.png
and on the test server homepage, like this:
http://i1146.photobucket.com/albums/...psc189f664.png

It looked fine when I was using Chrome, my usual browser, so I can only assume it's an Opera incompatibility.

I can still click on the empty circle where the button should be, to perform the action. Occasionally (about 10% of the time that I've tested it), hovering over the empty circle will cause the button to 'jump' back into place.