This is a confusing subject for most people at first, but an important one to understand. Here's an alternative explanation:
The raster is not the image, but instead the 'path' the electron beam follows from left to right, top to bottom in order to paint an image on the screen:
http://www.atomica.com/content2/img/CDE/RASTSCAN.GIF
The electron beam can be turned on during the black portions of the image above to paint an image. (The image is limited to the raster area - you can't have an image that's outside the raster area - that makes no sense).
CRT's use this method whereby electrons are shot onto the phosphor coating on the tube face a line at a time from left to right starting at the top-left corner (black lines). At the end of the line, the beam is turned off and moved back to the left and down one line, which is known as the horizontal retrace (flyback - red lines). When the bottom-right corner is reached, a vertical retrace (flyback - blue line) returns the gun to the top-left corner. In a TV signal, this is known as the vertical blanking interval.
As Tim stated, if you want to see the raster in order to move it or size it (this will also move the visible image), you can crank down the contrast (white level) close to zero and crank up the brightness (black level) while using an all black test pattern.
Some hints: Often the raster is somewhat 'unstable' after the retrace (red & blue lines). This can result in the left side of the image being wavy near the top left of the image So often people will shift the image to use the right-most area of rasterto allow some space on the left of the raster for the signal to stabilize first.
Clear as mud?
Kal