Adobe Ideas, iDraw, Sketchup and Inkpad all allow you to import or paste photos and draw over them. iDraw, Sketchup and Inkpad have vector freehand line tools that convert into an editable vector line. Adobe Ideas smooths out a line very nicely that you draw but the resultant object is not a vector but a raster line. Inkpad seems to have the most "Illustrator" like drawing tools. iDraw and Sketchup are more CAD like apps. They all have merits in different ways. I think Adobe Ideas would be the easiest app to trace over a photo but it has very limited tools and not vector as I said. iDraw has a very interesting variable width (calligraphy like) Brush tool as well as an adjustable width freehand line tool that can be used to trace.