GUI.Toolbar with different images for each button using GUIStyle

Yes,

One way is to have an array of textures:

public var toolbarImages : Texture[];

Set these in the Inspector and they will show over your buttons on the toolbar. Mess with your GUISkin Styles to get to display how you want.

I reccomend setting up a custom style and applying it to the toolbar, in which you remove the background and can tweak to your hearts content without messing up any of your other GUI buttons. Something like this, where customGuiStyle is the name of the custom style set up in your GUISkin:

GUILayout.Toolbar (toolbarInt, toolbarImages, "customGuiStyle");

Hope this helps.