There were several times I used to browse using Opera Mobile and the small Menu icon on the bottom, even though it has an initial transparency added, used to cover part of an image or text and I needed to scroll away in order to properly read what was written below it.

You can now replace the small Menu button with a fully transparent one that allows you to see through it. Just download the cab and install it to your device, it will replace the Opera skin with a new one which only has the Menu button modified.

Further below in the thread you will find another skin file which has both the Menu button and the Zoom button transparent for those using the zoom slider (I personally pinch zoom in Opera).

(Source: XDA Developers)