In addition to what JustG said - importing the image as a tracing paper so ArtRage chooses the colour as you do the brush strokes, you can also import the image as paint, and only brush in the bits you want.
So... start ArtRage.
From the 'file' menu, choose 'new painting'.
In the new painting dialog, in the second pane down, you'll see 'tracing image' - click 'Load tracing image'.
Select the photograph you want to use, and click the 'Use tracing image size' at the top of the new painting dialog.
Click 'OK' to create the new painting, with the tracing paper, at the same size as the original photograph.
Now, if you want to just paint in the whole picture, select the brush tool, choose a size of brush proportional to the detail you want to paint in, and start painting!
But if you want to do a bit more...
From the layers palette you can open the layers menu by clicking the small arrow just to the right of 'edit tracing image'.
From that menu, select 'convert tracing image to paint on this layer'. That fills the layer with paint, exactly the same as the tracing image.
Now when you do brush strokes, they mix with the colour already down. And you can paint as much or as little as you like.
Tip: Pressing the 'T' key shows and hides the tracing paper, so you can quickly see what you're doing more clearly as you work.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"