I routinely use bit maps for sketches. I use Corel Photo Paint but Photoshop or another photo editing software should work. What I do is open the bitmap in the photo software. I then convert the image to black & white. Next I do an auto trace of the image. If the image is very complex a manual track will be required. I then export the trace image as a DXF. Then you can insert it into SolidWorks just like any other DXF file.