hmm good question

Perhaps create a GDI Graphics object(dont know the namespace off the top of my head) that the Function can return

Code:
<Webmethod()>Public Function myImage() As Graphics

End Function
Do some searching for GDI+ to find info on loading an Image to the Graphics object and whatever namespaces needed

IO namespaces/classes might be able to be used for other files..
Cant be 100% sure on this though.