Open a sketch, then insert a Sketch Picture. The picture is represented in the tree as a child of the sketch. The picture can be suppressed independently from the sketch. If you use a sketch picture you have the option of leaving the sketch otherwise empty (no sketch entities other than the picture), which I believe is the only time you can do this. Often I use a separate sketch to actually trace just so the sketch with the picture doesn't get swallowed up by another feature. I usually name the sketch picture sketch something significant so it's easy to find later. It also (for me anyway) goes at or near the top of the tree.
It is best if the image is high contrast, so even zoomed up the edges look well defined. Anti aliasing in images makes the edges look a bit fuzzy when zoomed in and you may have difficulty placing a spline over it.
If you're tracing with splines, that takes a bit of practice. The spline curve itself is interpolated between the spline points, so the final shape of the curve at a point isn't finished until you have put down the next 2-3 points. If you have areas where the curvature is much tighter than the rest of the curve, you may want a higher density of spline points in that area. Generally speaking, when placing the points by hand, the fewer points you can use and still get the shape you want, the better.
Anyway, good luck.
Matt