Here's the trick:
Scan your image to make it .jpg, .tif, etc. Then, create a flat surface the same size as the sketch. If it's on an 8.5x11 piece of paper and you scanned it at 100%, sketch an 8.5x11 rectangular surface.
Next, go into View>Colors & Appearances, and create a new color. Under the Map tab add a decal to it. Load the .jpg as the decal and size it so that it exactly covers the flat surface.
You now have the artwork in ProE at scale. You can use this surface as your sketch plane and draw overtop the entities. Of course, it's still all free-hand since you can reference the image.......but it's better than nothing! I've done it several times to bring things like safety warning labels in - the ones where they show flames or a hand or a person. I would generate a sketched datum curve over it with all the sketches you need to build the part, because once you add that first feature, the decal-ed surface will be consumed by it, whereas you will always see the yellow lines of the datum curve.
Regards