For those of you folks hard at work building SharePoint Apps in the new SharePoint 2013 Preview App Platform, just an FYI that the TitleIconImageUrl attribute is not supported on the ClientWebPart is not supported in the Preview version of SharePoint 2013 at this time.
The TitleIconImageUrl is the little icon that is associated with your web part in the title bar or the add web parts view normally through the elements.xml file or maybe the app.manifest.xml. I’m sure that the folks at Microsoft will be working hard to ensure that this is fixed for RTM. For now we have to live with the default ugly icon.