Draw a large triangle. Leave
a white border (do not go edge to edge) and
place it high enough on the page to allow the addition of a tree base.
Switch to the rectangle shape and
draw a rectangle centered beneath the tree shape, leaving a space between
the tree and the base as shown.
Click on the Vector Object Selector
and drag a square over both objects, go from edge to edge so that there
is one square selection covering both images.
Go to the menu and select Objects
|| Group. Again go to Objects || Align || Horizontal Center in Canvas.
Convert your vector layer to raster,
by right-clicking on the vector layer in the layer's pallet and choosing
"convert to raster".