With bitmaps it's all about the quality of the image. Most images (but not all) on the web are heavily compressed to help with download times and look fine to the eye but if you zoom in to the edges what looks like a crisp line is a jumble of small squares.
For simple shapes like snowflakes you will get much better results by using the drawing tools to trace the picture by hand. As snowflakes are symmetrical shapes you would only need to draw one section and then copy the rest.
There are some good videos on auto tracing in the tips and tricks section - http://support.vectric.com/tips-and-tricks/select.php