D1-TPaintBox Help Needed-Nothing Draws!

More in my continuing saga...
     So I've got this object, descended from a Timage. I want this object
to appear when a user clicks the mouse in a certain part of a form. I
currently have code in the OnMouseUp event, (although I realize that
OnClick will be preferrable...), that creates an instance of my descended
object. So by using Form1.canvas.draw(X,Y,MyObjInstance); I get my object
at the point of the mouseUp. Cool.
     But let's say I don't want this to happen in the form as a whole, but
just within a certain area. OK, great, sounds like a TPaintBox to me. So I
drop that puppy on there, but now
Form1.PaintBox1.Canvas.Draw(X,Y,MyObjInstance) doesn't do anything. The
Online help is weak for the PaintBox, talking about both a Canvas
property, and the OnPaint event. I've tried to figure out code to put in
OnPaint, but then I have to change OnPaint's parameters to include my X,Y
values, and I don't know if it's kosher to change EventHandler parameters
so drastically. The fact that Delphi doesn't require any code in any event
handlers I would think I'm OK, but....And it also doesn't like me to
reference the Picture.bitmap property of the passed MyObjInstance with
sender.picture.bitmap. I would guess this is due to the fact that I
haven't published any properties of My created object, thus there isn't
the appropriate RTTI.
     And once I get this solved, then I'm going to want to put the
PaintBox within a ScrollBox!!! Ouch. Well, we'll cross that bridge later.
     Anyone got any ideas about how I can get my PaintBox to draw what I
want, where I want?
     Any Ideas appreciated,

Brad Miller