The ‘Magic Wand‘ tool allows you to automatically create a vector boundary around sections of imported artwork or imported relief models. Furthermore, you can adjust the tolerance of your selection to determine how closely you wish to follow the edge of the selection.
If the image you're working on has many colours, try using the Reduce Colors slider bar ('Bitmap > Colours > Reduce Number') to reduce the overall number of colours in the image while retaining a good amount of fidelity. This will aid the Magic Wand in creating clean vector boundaries.
You won't get a good result from photographs as these contain thousands of different colours and shades.