Why is it so important to have a timer that runs on 100fps?
IMO all that matters is how fluent a game runs. And you can have that even at 30fps.

If I understand you correctly, what you are looking for is a crop function, not a resize. You can do so by drawing a portion of the image. When drawing the surface, dont use clientrect, but your own defined rect.
ie not
Code:
DXDraw1.Surface.Draw(0,0, yourImageSurface.ClientRect, yourImageSurface, false);
but
Code:
DXDraw1.Surface.Draw(0,0, rect(10,10,50,50), yourImageSurface, false);