How to define custom animation UI for extjs message box

I need to pop up a message box in extjs with custom UI. I have tried Ext.MessageBox.show and Ext.Msg.wait methods for this purpose. What I really need to do is to show a custom "Loading" image in place of infinitely running rectangular progress bar. Is there any way, I can do it? or should I take a different approach?

Please pardon my开发者_运维问答 ignorance, I am a C++ programmer doing JS for the first time.

Try:

Ext.Msg.show({
    cls: "my-progress-class"
});

And define a CSS rule to show your custom "please wait" image:

.my-progress-class {
    background: url(image/wait.png) 50% 50% no-repeat;
}