I need to crop this .jpeg image:
in Photoshop CS2 to a vertical 1px image so I can repeatedly render it horizontally to create the full gradients that are dynamically sized.
Do you know how I could do this?
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Single column marquee tool found under (by default) the regular
Rectangular marquee tool in the toolbar to select 1 px wide and "image height" high area. 
Single row marguee tool works in a similar fashion. It selects an area that is 1 px high and "image width" wide.
Go to Image -> Canvas Size..., change the units to pixels, and change the width to 1 px. Photoshop will yell at you about the new dimensions being smaller than the original; click the "Yes" button.
Zoom to the maximum, such as 3200%, and it will show you the 1 pixel of the image. Then crop it from anywhere you want.