TMC,
Join the club. You can disable or remove some or all of those buttons, but you certainly cannot replace them with your own. Those are embedded in LabVIEW.
I would suggest that you disable the toolbar, and create your own. I tried to make a toolset for this, but implementation was impossible, as there is no way to either create dynamic buttons with custom icons, or embed custom icons on standing buttons. Everything must be done manually.
Good luck.