It not crystal clear what you mean.
You can load an image into Visio and resize or do whatever you want with it (Insert -> Picture -> From File).
To make it a shape go to (File -> Shapes -> New Stencil) to create a new stencil and simply drag the image on there. You now have an image available as a shape.
If you mean to transform your image to a scalable graphic, you could use a free program as inkscape and use (with Inkscape) Shift-Alt-B to trace the image as a scalable graphic.
