It has been a while since I have done this, and I thought that I could add a picture, but looking at my templates the picture was set in the form as a background and different forms had different pictures.
I am guessing thought that with code you could use another picture control to replace the background picture.
Have a picture control that lets them attach a picture.
Once it has a value hide and replace the stock picture in the ink control with the newly attached one via code.
Make the Ink picture visible.
Since it is saved in the template itself though I am not sure.
http://search.msdn.microsoft.com/search/Default.aspx?brand=msdn&query=ink%20picture%20infopath
Here is a link on MSDN but I did not see anyone had done this or even Identified the limitation. In one they give an example of an adjuster drawing on a picture of a car but I guess it would have to be a stock photo.