Easy viewing is very important component of correct editing, especially while dealing with larger images. To view the whole picture we have to zoom out, to view minor details we are to zoom in; to see a picture as it really is, we have to use the actual size.

2. Click the button on the Toolbox to activate the Zoom Tool. By default your cursor looks like this . Click on the image to magnify the view. Note the zoom value now. Click several more times. Now image may appear jagged, but this is just the sizing function and not a problem with your image.
Note that you magnify the view and not the image itself.

3. By now you can view minor details but you can't see the entire image, because it is too large. Open the Navigator palette using the Window pull down menu. 
In the center of the palette you can see an image thumbnail; the visible portion of the image window area is shown in a red rectangle. Drag the rectangle across the thumbnail and notice the image in the image window updating (movie).

4. Click the Zoom Out button on the Options bar or hold down [Alt] key. Now your cursor looks like this . Click on the image to reduce the view. Again, select the Zoom In button on the Options bar and magnify the view.

NB Also remember the keyboard zoom shortcuts:
[Ctrl+"+"] - Zoom In
[Ctrl+"-"] - Zoom Out