You can paint the image in OnPaint event of the form.
Canvas.Draw(X, Y, ...);
EDIT: If your form is sizable, you can use
Canvas.StretchDraw();
EDIT2: However, adding TImage does require to remove all components and add them again. It is just TGraphicControl. It should be possible to add it, anchor it to form and change its Z-order (Move to Back).