How to add icons to the Opera browser's toolbar?
From Opera Help pages:

Customize toolbars

You can change the appearance of a toolbar by adding items, changing where or when it displays, changing the style and wrapping, or returning to its defaults. To do this, Ctrl-click the toolbar and select "Customize". The toolbar is highlighted by a yellow border and you can start customizing the toolbar in the following ways:

To add an item, such as a button or search field, select "Appearance" and in the resulting dialog, click "Buttons" and drag a new button from the list for any toolbar to the highlighted toolbar.
To change where the toolbar displays in the browser, select "Appearance" and change the Placement setting.
To change the style of the toolbar, for example, to use text rather than buttons or images, select "Appearance" and change the Style setting.
To change the wrapping of the toolbar, select "Appearance" and change the Wrapping setting.
To set the toolbar to display only when it is needed, select "Appearance" and check "Show only when needed". This is especially useful for the navigation bar.
To undo any changes you made and reset a toolbar to the default setting, select "Reset Toolbar to Its Defaults".
To remove an item from a toolbar, select "Remove from Toolbar".
To change the order in which toolbar buttons or icons appear, drag-and-drop the buttons or icons.
To change the size of the buttons on all toolbars, select Tools &gt; Appearance &gt; Skin and in the "Icon Size" field, select the percentage to scale the buttons.