Use geometry words to describe a way to separate triangles into other triangles?

Let us suppose we have a triangle ABC as shown in the attached image.

Now, let us we draw a  perpendicular line from the vertex A to the base BC. Let the line meets the base BC at D.

Then, this perpendicular line AD will separate the triangle  ABC into two right angle triangle ADB and triangle ADC.

"image"

step-by-step explanation:

in most of these problems, the notation is that the point with a "prime" is the image of the point without. that is, b' is the image of b.

here, there is no dilation or reflection. there is simply translation (with no direction shown). which figure you call the "preimage" and which one you call the "image" is up to you. as a matter of convention, the naming is as described above.