Since Inventor uses IPictureDisp, any developer making an addin will have to use code to convert to this format. Wouldn't it make sense to include this conversion directly in the Inventor API?
I know about the blog post with the class to convert: http://modthemachine.typepad.com/my_weblog/2012/02/bitmaps-without-vb6-icontoipicture.html
Every Inventor developer currently includes this code, or their own version of it. It might also be convenient to have Inventor auto-scale the image to the proper size (like it did in 2012, but does not in 2013). See this post for that discussion: http://forums.autodesk.com/t5/Autodesk-Inventor-Customization/Inventor-2013-addin-Icons-displays-too...
Better yet, just support icons natively...