How to apply background image to button using css

Use its CssClass property say you have button like following

.myBtn
{
   background:transparent url('myImage.png') no-repeat;
}

now apply this class in your button in CssClass property.

Use Image Button[^] control instead of CSS.

In addition, please refer:
Simple way to Design Tabs in ASP.NET[^]
This article will show how to Create Tabs in ASP.NET page using Buttons, MultiView Control and CSS

Try something like this in your CSS class

.buttons
{
 

 background: url(images/orange_button_bg.gif) no-repeat 25% 50% #FF8800;
}