The DesignCentral - Bitmap tab allows you to change the resolution of a bitmap.
Ensure that Proportional is checked to keep the horizontal and vertical resolutions the same.
Changing the resolution does not change the number of pixels in the bitmap; it merely changes how many pixels fit into an inch. As the resolution of a bitmap is increased, the area covered by the bitmap will decrease, because more pixels will fit into each square inch. Decreasing the resolution will cause the bitmap to cover a larger area.