object LargeImage {get; set;}

// programmatically activates the Edit Annotation tool by invoking the corresponding
// plugin. The tool must be enabled for the operation to succeed.
{
  var plugin = FrameworkApplication.GetPlugInWrapper("esri_editing_EditVerticesText");
  if (plugin.Enabled)
    ((ICommand)plugin).Execute(null);
}

{
  IPlugInWrapper wrapper = FrameworkApplication.GetPlugInWrapper("MyAddin_MyCustomButton");
  if (wrapper != null)
  {
    wrapper.Caption = "new caption";

    // ensure that T-Rex16 and T-Rex32 are included in your add-in under the images folder and have 
    // BuildAction = Resource and Copy to OutputDirectory = Do not copy
    wrapper.SmallImage = buildImage;
    wrapper.LargeImage = buildImage2;
  }
}