ok lets see....
1st: draw the line C to D until it touches the mirror
2nd:draw the line A to B until it touches the mirror
3rd: extend the lines CD and AB to the back of the mirror until they intersect
4th: the point of intersection(at the back of the mirror) is the place where the image position I is gonna be.
thats it
therefore it is virtual because the lines intersect behind the mirror
if it intersected in front of the mirror, then it is real...
do u get it??