fieldStyle

http://stackoverflow.com/questions/13771723/change-color-of-xtype-text-in-extjs4-1

HTML

<link rel="stylesheet" href="http://cdn.sencha.io/ext-4.1.0-gpl/resources/css/ext-all.css">

CSS

.question{
                                                                   background-image:url(http://www.southampton.ac.uk/isolutions/computing/elearn/blackboard/small_pen.gif)!important;
            background-repeat: no-repeat;
            
        }

JavaScript

Ext.tip.QuickTipManager.init();
Ext.create('Ext.form.Panel', {
    title: 'Contact Info',
    width: 300,
    bodyPadding: 10,
    renderTo: Ext.getBody(),
    items: [{
            xtype: 'button',
            iconCls:'question',
            tooltip:"<b>read-only</b>:Read-only users will have read only access to all pages<br> ",
            padding: '2 6 2 7'
                    
           }]
});