The most likely reason is that you were using a much older version of mIRC that was loading an old toolbar bitmap format. The toolbar bitmap format can change from version to version. This is one of the reasons why recent versions of mIRC added support for a /toolbar command, which you can use to customize your toolbar without having to rely on the ever-changing bitmap format that mIRC uses internally. To clear your current bitmap selection, right-click on the Toolbar to view the Toolbar popup menu, select "Buttons" and then "None".
