Here's a quick way to make a dotted line with Photoshop.

1. Select the Brush tool in the toolbox.

2. In the Brushes palette, click on Brush Tip Shape and select a brush tip. (Choose Window > Brushes if the palette isn't open.)

3. Change the spacing to any number greater than 100%, as shown in the image. (The number depends on how much space you want between the dots.)

4. Click and drag in the image to paint and you'll see the dotted line. To draw a straight line, click a starting point in the image. Then press Shift and click an ending point.