Jaboone,
Depending on what your goal is, there are different ways to go about creating your addin. I, personally, am building a full-scale addin for Revit, so rather than placing a button in their ribbon somewhere, I create my own ribbon tab. If you're just looking to create one tool, and have it be located in under the External Tools stacked button, you can find more about that here.
From my experience, the icon is a property of the button that you set when you create the button. I don't know how you're doing it, but if you're using the Autodesk.Revit.UI.RibbonPanel.AddItem(...) function that I use, it returns a PushButton instance that has a LargeImage property - assign a BitmapSource to it like so:
//Place an icon image (32x32) in your resources, and copy it to the Properties::Resources
IntPtr pIcon = Properties.Resources.MyButtonIcon.GetHBitmap();
BitmapSource pIconBmp = Imaging.CreateBitmapSourceFromHBitmap(pIcon, IntPtr.Zero, System.Windows.Int32Rect.Empty, BitmapSizeOptions.FromEmptyOptions());
pBtn.LargeImage = pIconBmp;
If you're looking at putting together a more elaborate addin that would require your own ribbon panel, like the image below, I can put together something to help with that. Let me know.

Don't forget to mark your answers... It helps the next individual find theirs faster!!
Cheers,
Geoff Overfield
Software Engineer
FabPro, LLC
